Merge pull request #1042 from Invincibear/patch-6

Add support for multiple Prefer headers in Connection class

O365/connection.py

@@ -719,14 +719,16 @@ def _internal_request(self, request_obj, url, method, **kwargs):
         """
         method = method.lower()
         if method not in self._allowed_methods:
-            raise ValueError('Method must be one of the allowed ones')
+            raise ValueError('Method must be one of: {}'.format(self._allowed_methods))
 
         if 'headers' not in kwargs:
             kwargs['headers'] = {**self.default_headers}
         else:
             for key, value in self.default_headers.items():
                 if key not in kwargs['headers']:
                     kwargs['headers'][key] = value
+                elif key == 'Prefer' and key in kwargs['headers']:
+                    kwargs['headers'][key] = "{}, {}".format(kwargs['headers'][key], value)
 
         if method == 'get':
             kwargs.setdefault('allow_redirects', True)