Uploads fail when there's no Content-Length header

[turt2live]

This header is not required.

2019-06-03 09:10:14,485 - synapse.http.server - 112 - ERROR - POST-23730- Failed handle request via 'UploadResource': <XForwardedForRequest at 0x7ff60923acc0 method='POST' uri='/_matrix/media/v1/upload?filename=image001.png&access_token=<redacted>' clientproto='HTTP/1.1' site=8008>
Traceback (most recent call last):
  File "/opt/venvs/matrix-synapse/lib/python3.6/site-packages/synapse/http/server.py", line 81, in wrapped_request_handler
    yield h(self, request)
  File "/opt/venvs/matrix-synapse/lib/python3.6/site-packages/twisted/internet/defer.py", line 1418, in _inlineCallbacks
    result = g.send(result)
  File "/opt/venvs/matrix-synapse/lib/python3.6/site-packages/synapse/rest/media/v1/upload_resource.py", line 58, in _async_render_POST
    content_length = request.getHeader(b"Content-Length").decode('ascii')
AttributeError: 'NoneType' object has no attribute 'decode'

[richvdh]

dup #5114