integrations: Broaden return type of check_send_webhook_fixture_message.
integrations: Broaden return type of check_send_webhook_fixture_message.

With the new signature of has_request_variables, we can now use
`HttpResponseBase` as the return type of the decorated function.

Signed-off-by: Zixuan James Li <74923aee05c23ca90566381b5d6b0cd9451b6054@gmail.com>

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