Fix dynamic model getattr (#764)

Author: Jason Munro

mp_api/client/core/client.py

@@ -759,7 +759,7 @@ def new_str(self) -> str:
             return s
 
         def new_getattr(self, attr) -> str:
-            if attr in unset_fields:
+            if attr in self.fields_not_requested:
                 raise AttributeError(
                     f"'{attr}' data is available but has not been requested in 'fields'."
                     " A full list of unrequested fields can be found in `fields_not_requested`."