Modify _get_digest to use request.get_data().

* request.data is empty when the request type is application/x-www-form-urlencoded
* request.get_data contains the raw request bytes, cached by flask, and
  that works for both application/json and application/x-www-form-urlencoded

Signed-Off-By: Ahmed Awad <ahmed.awad.15@ucl.ac.uk>

Author: Ahmed Awad

github_webhook/webhook.py

@@ -42,7 +42,7 @@ def decorator(func):
     def _get_digest(self):
         """Return message digest if a secret key was provided"""
 
-        return hmac.new(self._secret, request.data, hashlib.sha1).hexdigest() if self._secret else None
+        return hmac.new(self._secret, request.get_data(), hashlib.sha1).hexdigest() if self._secret else None
 
     def _postreceive(self):
         """Callback from Flask"""