middleware: Assert request.method is not None.
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 <74923aee05c23ca90566381b5d6b0cd9451b6054@gmail.com>

zulip/zulip,rht/zulip,andersk/zulip,andersk/zulip,zulip/zulip,zulip/zulip,rht/zulip,rht/zulip,zulip/zulip,zulip/zulip,rht/zulip,andersk/zulip,andersk/zulip,rht/zulip,andersk/zulip,zulip/zulip,andersk/zulip,zulip/zulip,rht/zulip,rht/zulip,andersk/zulip