We are currently solving the following issue within our repository. Here is the issue text:
--- BEGIN ISSUE ---
Default OPTIONS responder does not set Content-Length to "0"
Per RFC 7231:

> A server MUST generate a Content-Length field with a value of "0" if no payload body is to be sent in the response.


--- END ISSUE ---

Below are some code segments, each from a relevant file. One or more of these files may contain bugs.
--- BEGIN FILES ---
Path: `falcon/responders.py`
Content:
```
1 # Copyright 2013 by Rackspace Hosting, Inc.
2 #
3 # Licensed under the Apache License, Version 2.0 (the "License");
4 # you may not use this file except in compliance with the License.
5 # You may obtain a copy of the License at
6 #
7 #    http://www.apache.org/licenses/LICENSE-2.0
8 #
9 # Unless required by applicable law or agreed to in writing, software
10 # distributed under the License is distributed on an "AS IS" BASIS,
11 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
12 # See the License for the specific language governing permissions and
13 # limitations under the License.
14 
15 from falcon.errors import HTTPBadRequest
16 from falcon.errors import HTTPNotFound
17 from falcon.status_codes import HTTP_204
18 from falcon.status_codes import HTTP_405
19 
20 
21 def path_not_found(req, resp, **kwargs):
22     """Raise 404 HTTPNotFound error"""
23     raise HTTPNotFound()
24 
25 
26 def bad_request(req, resp, **kwargs):
27     """Raise 400 HTTPBadRequest error"""
28     raise HTTPBadRequest('Bad request', 'Invalid HTTP method')
29 
30 
31 def create_method_not_allowed(allowed_methods):
32     """Creates a responder for "405 Method Not Allowed"
33 
34     Args:
35         allowed_methods: A list of HTTP methods (uppercase) that should be
36             returned in the Allow header.
37 
38     """
39     allowed = ', '.join(allowed_methods)
40 
41     def method_not_allowed(req, resp, **kwargs):
42         resp.status = HTTP_405
43         resp.set_header('Allow', allowed)
44 
45     return method_not_allowed
46 
47 
48 def create_default_options(allowed_methods):
49     """Creates a default responder for the OPTIONS method
50 
51     Args:
52         allowed_methods: A list of HTTP methods (uppercase) that should be
53             returned in the Allow header.
54 
55     """
56     allowed = ', '.join(allowed_methods)
57 
58     def on_options(req, resp, **kwargs):
59         resp.status = HTTP_204
60         resp.set_header('Allow', allowed)
61 
62     return on_options
63
```
--- 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/falcon/responders.py b/falcon/responders.py
--- a/falcon/responders.py
+++ b/falcon/responders.py
@@ -58,5 +58,6 @@
     def on_options(req, resp, **kwargs):
         resp.status = HTTP_204
         resp.set_header('Allow', allowed)
+        resp.set_header('Content-Length', '0')
 
     return on_options

{"golden_diff": "diff --git a/falcon/responders.py b/falcon/responders.py\n--- a/falcon/responders.py\n+++ b/falcon/responders.py\n@@ -58,5 +58,6 @@\n     def on_options(req, resp, **kwargs):\n         resp.status = HTTP_204\n         resp.set_header('Allow', allowed)\n+        resp.set_header('Content-Length', '0')\n \n     return on_options\n", "issue": "Default OPTIONS responder does not set Content-Length to \"0\"\nPer RFC 7231:\n\n> A server MUST generate a Content-Length field with a value of \"0\" if no payload body is to be sent in the response.\n\n", "before_files": [{"content": "# Copyright 2013 by Rackspace Hosting, Inc.\n#\n# Licensed under the Apache License, Version 2.0 (the \"License\");\n# you may not use this file except in compliance with the License.\n# You may obtain a copy of the License at\n#\n#    http://www.apache.org/licenses/LICENSE-2.0\n#\n# Unless required by applicable law or agreed to in writing, software\n# distributed under the License is distributed on an \"AS IS\" BASIS,\n# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.\n# See the License for the specific language governing permissions and\n# limitations under the License.\n\nfrom falcon.errors import HTTPBadRequest\nfrom falcon.errors import HTTPNotFound\nfrom falcon.status_codes import HTTP_204\nfrom falcon.status_codes import HTTP_405\n\n\ndef path_not_found(req, resp, **kwargs):\n    \"\"\"Raise 404 HTTPNotFound error\"\"\"\n    raise HTTPNotFound()\n\n\ndef bad_request(req, resp, **kwargs):\n    \"\"\"Raise 400 HTTPBadRequest error\"\"\"\n    raise HTTPBadRequest('Bad request', 'Invalid HTTP method')\n\n\ndef create_method_not_allowed(allowed_methods):\n    \"\"\"Creates a responder for \"405 Method Not Allowed\"\n\n    Args:\n        allowed_methods: A list of HTTP methods (uppercase) that should be\n            returned in the Allow header.\n\n    \"\"\"\n    allowed = ', '.join(allowed_methods)\n\n    def method_not_allowed(req, resp, **kwargs):\n        resp.status = HTTP_405\n        resp.set_header('Allow', allowed)\n\n    return method_not_allowed\n\n\ndef create_default_options(allowed_methods):\n    \"\"\"Creates a default responder for the OPTIONS method\n\n    Args:\n        allowed_methods: A list of HTTP methods (uppercase) that should be\n            returned in the Allow header.\n\n    \"\"\"\n    allowed = ', '.join(allowed_methods)\n\n    def on_options(req, resp, **kwargs):\n        resp.status = HTTP_204\n        resp.set_header('Allow', allowed)\n\n    return on_options\n", "path": "falcon/responders.py"}], "after_files": [{"content": "# Copyright 2013 by Rackspace Hosting, Inc.\n#\n# Licensed under the Apache License, Version 2.0 (the \"License\");\n# you may not use this file except in compliance with the License.\n# You may obtain a copy of the License at\n#\n#    http://www.apache.org/licenses/LICENSE-2.0\n#\n# Unless required by applicable law or agreed to in writing, software\n# distributed under the License is distributed on an \"AS IS\" BASIS,\n# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.\n# See the License for the specific language governing permissions and\n# limitations under the License.\n\nfrom falcon.errors import HTTPBadRequest\nfrom falcon.errors import HTTPNotFound\nfrom falcon.status_codes import HTTP_204\nfrom falcon.status_codes import HTTP_405\n\n\ndef path_not_found(req, resp, **kwargs):\n    \"\"\"Raise 404 HTTPNotFound error\"\"\"\n    raise HTTPNotFound()\n\n\ndef bad_request(req, resp, **kwargs):\n    \"\"\"Raise 400 HTTPBadRequest error\"\"\"\n    raise HTTPBadRequest('Bad request', 'Invalid HTTP method')\n\n\ndef create_method_not_allowed(allowed_methods):\n    \"\"\"Creates a responder for \"405 Method Not Allowed\"\n\n    Args:\n        allowed_methods: A list of HTTP methods (uppercase) that should be\n            returned in the Allow header.\n\n    \"\"\"\n    allowed = ', '.join(allowed_methods)\n\n    def method_not_allowed(req, resp, **kwargs):\n        resp.status = HTTP_405\n        resp.set_header('Allow', allowed)\n\n    return method_not_allowed\n\n\ndef create_default_options(allowed_methods):\n    \"\"\"Creates a default responder for the OPTIONS method\n\n    Args:\n        allowed_methods: A list of HTTP methods (uppercase) that should be\n            returned in the Allow header.\n\n    \"\"\"\n    allowed = ', '.join(allowed_methods)\n\n    def on_options(req, resp, **kwargs):\n        resp.status = HTTP_204\n        resp.set_header('Allow', allowed)\n        resp.set_header('Content-Length', '0')\n\n    return on_options\n", "path": "falcon/responders.py"}]}