fix: Make is_filerable work well by using callable()

Author: abawchen

graphene_mongo/fields.py

@@ -66,7 +66,8 @@ def args(self, args):
     @property
     def default_filter_args(self):
         def is_filterable(kv):
-            return hasattr(kv[1], '_type') and hasattr(kv[1]._type, '_of_type')
+            return hasattr(kv[1], '_type') \
+                    and callable(getattr(kv[1]._type, '_of_type', None))
 
         return reduce(
             lambda r, kv: r.update({kv[0]: kv[1]._type._of_type()}) or r if is_filterable(kv) else r,