middleware: Assert request.method is not None.

`request.method` is not `None` in normal use cases, unless an
`HttpRequest` is directly instantiated without the method being set.
This situation does not apply to `WSGIRequest` at all.

Signed-off-by: Zixuan James Li <p359101898@gmail.com>

zerver/middleware.py

@@ -434,6 +434,7 @@ class LogRequests(MiddlewareMixin):
             content_iter = None
 
         assert request_notes.client_name is not None and request_notes.log_data is not None
+        assert request.method is not None
         write_log_line(
             request_notes.log_data,
             request.path,