We are currently solving the following issue within our repository. Here is the issue text:
--- BEGIN ISSUE ---
Is HTTPHeaderDict a public API to make requests?
`HTTPHeaderDict` was initially designed to handle response headers, and is not documented for request headers.

 * Should it be documented?
 * How should it be imported? The current options are `from urllib3.response import HTTPHeaderDict` and `from urllib3._collections import HTTPHeaderDict`, and they don't feel right.

--- END ISSUE ---

Below are some code segments, each from a relevant file. One or more of these files may contain bugs.
--- BEGIN FILES ---
Path: `src/urllib3/__init__.py`
Content:
```
1 """
2 Python HTTP library with thread-safe connection pooling, file post support, user friendly, and more
3 """
4 
5 # Set default logging handler to avoid "No handler found" warnings.
6 import logging
7 import warnings
8 from logging import NullHandler
9 
10 from . import exceptions
11 from ._version import __version__
12 from .connectionpool import HTTPConnectionPool, HTTPSConnectionPool, connection_from_url
13 from .filepost import encode_multipart_formdata
14 from .poolmanager import PoolManager, ProxyManager, proxy_from_url
15 from .response import HTTPResponse
16 from .util.request import make_headers
17 from .util.retry import Retry
18 from .util.timeout import Timeout
19 
20 __author__ = "Andrey Petrov (andrey.petrov@shazow.net)"
21 __license__ = "MIT"
22 __version__ = __version__
23 
24 __all__ = (
25     "HTTPConnectionPool",
26     "HTTPSConnectionPool",
27     "PoolManager",
28     "ProxyManager",
29     "HTTPResponse",
30     "Retry",
31     "Timeout",
32     "add_stderr_logger",
33     "connection_from_url",
34     "disable_warnings",
35     "encode_multipart_formdata",
36     "make_headers",
37     "proxy_from_url",
38     "request",
39 )
40 
41 logging.getLogger(__name__).addHandler(NullHandler())
42 
43 
44 def add_stderr_logger(level=logging.DEBUG):
45     """
46     Helper for quickly adding a StreamHandler to the logger. Useful for
47     debugging.
48 
49     Returns the handler after adding it.
50     """
51     # This method needs to be in this __init__.py to get the __name__ correct
52     # even if urllib3 is vendored within another package.
53     logger = logging.getLogger(__name__)
54     handler = logging.StreamHandler()
55     handler.setFormatter(logging.Formatter("%(asctime)s %(levelname)s %(message)s"))
56     logger.addHandler(handler)
57     logger.setLevel(level)
58     logger.debug("Added a stderr logging handler to logger: %s", __name__)
59     return handler
60 
61 
62 # ... Clean up.
63 del NullHandler
64 
65 
66 # All warning filters *must* be appended unless you're really certain that they
67 # shouldn't be: otherwise, it's very hard for users to use most Python
68 # mechanisms to silence them.
69 # SecurityWarning's always go off by default.
70 warnings.simplefilter("always", exceptions.SecurityWarning, append=True)
71 # InsecurePlatformWarning's don't vary between requests, so we keep it default.
72 warnings.simplefilter("default", exceptions.InsecurePlatformWarning, append=True)
73 # SNIMissingWarnings should go off only once.
74 warnings.simplefilter("default", exceptions.SNIMissingWarning, append=True)
75 
76 
77 def disable_warnings(category=exceptions.HTTPWarning):
78     """
79     Helper for quickly disabling all urllib3 warnings.
80     """
81     warnings.simplefilter("ignore", category)
82 
83 
84 _DEFAULT_POOL = PoolManager()
85 
86 
87 def request(method, url, fields=None, headers=None):
88     """
89     A convenience, top-level request method. It uses a module-global ``PoolManager`` instance.
90     Therefore, its side effects could be shared across dependencies relying on it.
91     To avoid side effects create a new ``PoolManager`` instance and use it instead.
92     The method does not accept low-level ``**urlopen_kw`` keyword arguments.
93     """
94 
95     return _DEFAULT_POOL.request(method, url, fields=fields, headers=headers)
96
```
--- END FILES ---

Please first localize the bug based on the issue statement, and then generate a patch according to the `git diff` format fenced by three backticks.

Here is an example:

```diff
diff --git a/examples/server_async.py b/examples/server_async.py
--- a/examples/server_async.py
+++ b/examples/server_async.py
@@ -313,4 +313,4 @@
 
 
 if __name__ == "__main__":
-    asyncio.run(run_async_server("."), debug=True)
+    asyncio.run(run_async_server(), debug=True)
diff --git a/examples/server_sync.py b/examples/server_sync.py
--- a/examples/server_sync.py
+++ b/examples/server_sync.py
@@ -313,5 +313,5 @@
 
 
 if __name__ == "__main__":
-    server = run_sync_server(".")
+    server = run_sync_server()
     server.shutdown()

```

diff --git a/src/urllib3/__init__.py b/src/urllib3/__init__.py
--- a/src/urllib3/__init__.py
+++ b/src/urllib3/__init__.py
@@ -8,6 +8,7 @@
 from logging import NullHandler
 
 from . import exceptions
+from ._collections import HTTPHeaderDict
 from ._version import __version__
 from .connectionpool import HTTPConnectionPool, HTTPSConnectionPool, connection_from_url
 from .filepost import encode_multipart_formdata
@@ -23,6 +24,7 @@
 
 __all__ = (
     "HTTPConnectionPool",
+    "HTTPHeaderDict",
     "HTTPSConnectionPool",
     "PoolManager",
     "ProxyManager",

{"golden_diff": "diff --git a/src/urllib3/__init__.py b/src/urllib3/__init__.py\n--- a/src/urllib3/__init__.py\n+++ b/src/urllib3/__init__.py\n@@ -8,6 +8,7 @@\n from logging import NullHandler\n \n from . import exceptions\n+from ._collections import HTTPHeaderDict\n from ._version import __version__\n from .connectionpool import HTTPConnectionPool, HTTPSConnectionPool, connection_from_url\n from .filepost import encode_multipart_formdata\n@@ -23,6 +24,7 @@\n \n __all__ = (\n     \"HTTPConnectionPool\",\n+    \"HTTPHeaderDict\",\n     \"HTTPSConnectionPool\",\n     \"PoolManager\",\n     \"ProxyManager\",\n", "issue": "Is HTTPHeaderDict a public API to make requests?\n`HTTPHeaderDict` was initially designed to handle response headers, and is not documented for request headers.\r\n\r\n * Should it be documented?\r\n * How should it be imported? The current options are `from urllib3.response import HTTPHeaderDict` and `from urllib3._collections import HTTPHeaderDict`, and they don't feel right.\n", "before_files": [{"content": "\"\"\"\nPython HTTP library with thread-safe connection pooling, file post support, user friendly, and more\n\"\"\"\n\n# Set default logging handler to avoid \"No handler found\" warnings.\nimport logging\nimport warnings\nfrom logging import NullHandler\n\nfrom . import exceptions\nfrom ._version import __version__\nfrom .connectionpool import HTTPConnectionPool, HTTPSConnectionPool, connection_from_url\nfrom .filepost import encode_multipart_formdata\nfrom .poolmanager import PoolManager, ProxyManager, proxy_from_url\nfrom .response import HTTPResponse\nfrom .util.request import make_headers\nfrom .util.retry import Retry\nfrom .util.timeout import Timeout\n\n__author__ = \"Andrey Petrov (andrey.petrov@shazow.net)\"\n__license__ = \"MIT\"\n__version__ = __version__\n\n__all__ = (\n    \"HTTPConnectionPool\",\n    \"HTTPSConnectionPool\",\n    \"PoolManager\",\n    \"ProxyManager\",\n    \"HTTPResponse\",\n    \"Retry\",\n    \"Timeout\",\n    \"add_stderr_logger\",\n    \"connection_from_url\",\n    \"disable_warnings\",\n    \"encode_multipart_formdata\",\n    \"make_headers\",\n    \"proxy_from_url\",\n    \"request\",\n)\n\nlogging.getLogger(__name__).addHandler(NullHandler())\n\n\ndef add_stderr_logger(level=logging.DEBUG):\n    \"\"\"\n    Helper for quickly adding a StreamHandler to the logger. Useful for\n    debugging.\n\n    Returns the handler after adding it.\n    \"\"\"\n    # This method needs to be in this __init__.py to get the __name__ correct\n    # even if urllib3 is vendored within another package.\n    logger = logging.getLogger(__name__)\n    handler = logging.StreamHandler()\n    handler.setFormatter(logging.Formatter(\"%(asctime)s %(levelname)s %(message)s\"))\n    logger.addHandler(handler)\n    logger.setLevel(level)\n    logger.debug(\"Added a stderr logging handler to logger: %s\", __name__)\n    return handler\n\n\n# ... Clean up.\ndel NullHandler\n\n\n# All warning filters *must* be appended unless you're really certain that they\n# shouldn't be: otherwise, it's very hard for users to use most Python\n# mechanisms to silence them.\n# SecurityWarning's always go off by default.\nwarnings.simplefilter(\"always\", exceptions.SecurityWarning, append=True)\n# InsecurePlatformWarning's don't vary between requests, so we keep it default.\nwarnings.simplefilter(\"default\", exceptions.InsecurePlatformWarning, append=True)\n# SNIMissingWarnings should go off only once.\nwarnings.simplefilter(\"default\", exceptions.SNIMissingWarning, append=True)\n\n\ndef disable_warnings(category=exceptions.HTTPWarning):\n    \"\"\"\n    Helper for quickly disabling all urllib3 warnings.\n    \"\"\"\n    warnings.simplefilter(\"ignore\", category)\n\n\n_DEFAULT_POOL = PoolManager()\n\n\ndef request(method, url, fields=None, headers=None):\n    \"\"\"\n    A convenience, top-level request method. It uses a module-global ``PoolManager`` instance.\n    Therefore, its side effects could be shared across dependencies relying on it.\n    To avoid side effects create a new ``PoolManager`` instance and use it instead.\n    The method does not accept low-level ``**urlopen_kw`` keyword arguments.\n    \"\"\"\n\n    return _DEFAULT_POOL.request(method, url, fields=fields, headers=headers)\n", "path": "src/urllib3/__init__.py"}], "after_files": [{"content": "\"\"\"\nPython HTTP library with thread-safe connection pooling, file post support, user friendly, and more\n\"\"\"\n\n# Set default logging handler to avoid \"No handler found\" warnings.\nimport logging\nimport warnings\nfrom logging import NullHandler\n\nfrom . import exceptions\nfrom ._collections import HTTPHeaderDict\nfrom ._version import __version__\nfrom .connectionpool import HTTPConnectionPool, HTTPSConnectionPool, connection_from_url\nfrom .filepost import encode_multipart_formdata\nfrom .poolmanager import PoolManager, ProxyManager, proxy_from_url\nfrom .response import HTTPResponse\nfrom .util.request import make_headers\nfrom .util.retry import Retry\nfrom .util.timeout import Timeout\n\n__author__ = \"Andrey Petrov (andrey.petrov@shazow.net)\"\n__license__ = \"MIT\"\n__version__ = __version__\n\n__all__ = (\n    \"HTTPConnectionPool\",\n    \"HTTPHeaderDict\",\n    \"HTTPSConnectionPool\",\n    \"PoolManager\",\n    \"ProxyManager\",\n    \"HTTPResponse\",\n    \"Retry\",\n    \"Timeout\",\n    \"add_stderr_logger\",\n    \"connection_from_url\",\n    \"disable_warnings\",\n    \"encode_multipart_formdata\",\n    \"make_headers\",\n    \"proxy_from_url\",\n    \"request\",\n)\n\nlogging.getLogger(__name__).addHandler(NullHandler())\n\n\ndef add_stderr_logger(level=logging.DEBUG):\n    \"\"\"\n    Helper for quickly adding a StreamHandler to the logger. Useful for\n    debugging.\n\n    Returns the handler after adding it.\n    \"\"\"\n    # This method needs to be in this __init__.py to get the __name__ correct\n    # even if urllib3 is vendored within another package.\n    logger = logging.getLogger(__name__)\n    handler = logging.StreamHandler()\n    handler.setFormatter(logging.Formatter(\"%(asctime)s %(levelname)s %(message)s\"))\n    logger.addHandler(handler)\n    logger.setLevel(level)\n    logger.debug(\"Added a stderr logging handler to logger: %s\", __name__)\n    return handler\n\n\n# ... Clean up.\ndel NullHandler\n\n\n# All warning filters *must* be appended unless you're really certain that they\n# shouldn't be: otherwise, it's very hard for users to use most Python\n# mechanisms to silence them.\n# SecurityWarning's always go off by default.\nwarnings.simplefilter(\"always\", exceptions.SecurityWarning, append=True)\n# InsecurePlatformWarning's don't vary between requests, so we keep it default.\nwarnings.simplefilter(\"default\", exceptions.InsecurePlatformWarning, append=True)\n# SNIMissingWarnings should go off only once.\nwarnings.simplefilter(\"default\", exceptions.SNIMissingWarning, append=True)\n\n\ndef disable_warnings(category=exceptions.HTTPWarning):\n    \"\"\"\n    Helper for quickly disabling all urllib3 warnings.\n    \"\"\"\n    warnings.simplefilter(\"ignore\", category)\n\n\n_DEFAULT_POOL = PoolManager()\n\n\ndef request(method, url, fields=None, headers=None):\n    \"\"\"\n    A convenience, top-level request method. It uses a module-global ``PoolManager`` instance.\n    Therefore, its side effects could be shared across dependencies relying on it.\n    To avoid side effects create a new ``PoolManager`` instance and use it instead.\n    The method does not accept low-level ``**urlopen_kw`` keyword arguments.\n    \"\"\"\n\n    return _DEFAULT_POOL.request(method, url, fields=fields, headers=headers)\n", "path": "src/urllib3/__init__.py"}]}