Return the signed response when signing

If caller asked for a signed response, we would return None.
Return the response from self.sign() instead.

Author: bcopeland

src/saml2/entity.py

@@ -451,7 +451,7 @@ def _response(self, in_response_to, consumer_url=None, status=None,
         self._add_info(response, **kwargs)
 
         if sign:
-            self.sign(response, to_sign=to_sign)
+            return self.sign(response, to_sign=to_sign)
         elif to_sign:
             return signed_instance_factory(response, self.sec, to_sign)
         else: