Avoid popping from empty list

Refs #21

Author: ricardogsilva

asgi_csrf.py

@@ -242,7 +242,10 @@ async def _parse_form_urlencoded(receive):
         more_body = message.get("more_body", False)
 
     async def replay_receive():
-        return messages.pop(0)
+        if messages:
+            return messages.pop(0)
+        else:
+            return await receive()
 
     return dict(parse_qsl(body.decode("utf-8"))), replay_receive