<xml/>", "application/xml"); },
        {"Content-Type": "application/xhtml+xml;charset=ASCII"},
        "application/xhtml+xml;charset=UTF-8",
        "XML Document request keeps setRequestHeader() Content-Type, with charset adjusted to UTF-8"
      )
      request(
        function _HTMLDocument() { return new DOMParser().parseFromString("<html></html>", "text/html"); },
        {"Content-Type": ""},
        "",
        'HTML Document request respects setRequestHeader("")'
      )
      request(
        f