Allowed hooks to return own HTTP response

Hooks may need to return a 400 if the client has send an incorrect
payload.  The HTTP response generated by the hook is now returned
to the HTTP client iff the resposne is not None.

Author: Aidan Delaney

github_webhook/webhook.py

@@ -69,7 +69,9 @@ def _postreceive(self):
             '%s (%s)', _format_event(event_type, data), _get_header('X-Github-Delivery'))
 
         for hook in self._hooks.get(event_type, []):
-            hook(data)
+            resp = hook(data)
+            if resp:  # Allow hook to respond if necessary
+                return resp
 
         return '', 204