Return the callback result

Author: juannyG

mongoengine/connection.py

@@ -557,6 +557,6 @@ def callback(session, custom_arg, customer_kwarg=None):
         transaction_kwargs["callback"] = callback
         _set_session(session)
         try:
-            session.with_transaction(**transaction_kwargs)
+            return session.with_transaction(**transaction_kwargs)
         finally:
             _clear_session()