Fix regression breaking websocket message sending.

In fe1ba6f3eb, we change our auth
decorators etc. to use request.POST/request.GET rather than the (now
removed request.REQUEST).  This broke sending messages via our
websockets codepath, because we failed to update the artificial
requests we generate to use request._post as well.

zerver/worker/queue_processors.py

@@ -340,7 +340,8 @@ class MessageSenderWorker(QueueProcessingWorker):
         # We're mostly using a WSGIRequest for convenience
         environ.update(server_meta['request_environ'])
         request = WSGIRequest(environ)
-        request._request = event['request']
+        # Note: If we ever support non-POST methods, we'll need to change this.
+        request._post = event['request']
         request.csrf_processing_done = True
 
         user_profile = get_user_profile_by_id(server_meta['user_id'])