rest_dispatch: Remove invalid Content-Length: 0 from 204 responses.

There’s an apparent contradiction between RFC 7230 §3.3.2
Content-Length:

“A server MUST NOT send a Content-Length header field in any response
with a status code of 1xx (Informational) or 204 (No Content).”

and RFC 7231 §4.3.7 OPTIONS:

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

The only resolution within the existing language would be to disallow
all 204 responses to OPTIONS requests.  However, I don’t think that
was the intention, so I submitted this erratum report:

https://www.rfc-editor.org/errata/eid5806

and updated the code accordingly.

Signed-off-by: Anders Kaseorg <anders@zulipchat.com>

zerver/lib/rest.py

@@ -71,7 +71,6 @@ def rest_dispatch(request: HttpRequest, **kwargs: Any) -> HttpResponse:
     if request.method == 'OPTIONS':
         response = HttpResponse(status=204)  # No content
         response['Allow'] = ', '.join(sorted(supported_methods.keys()))
-        response['Content-Length'] = "0"
         return response
 
     # Override requested method if magic method=??? parameter exists