Fix result from mutation

Author: patrick91

graphene_django/rest_framework/mutation.py

@@ -124,4 +124,6 @@ def mutate(cls, instance, args, request, info):
 
     @classmethod
     def perform_mutate(cls, serializer, info):
-        return serializer.save()
+        obj = serializer.save()
+
+        return cls(**obj)