Fix missing related managers on some models (#902)

* Fix missing related managers on some models

I was seeing an issue where some related managers were missing from some
models. Traced the issue down to this line, where it appears that if we
hit a relation with a non-default(?) reverse manager the iteration
stopped. I _think_ this is supposed to be a continue statement instead.
It appears to work in the project I'm working in at least.

* Add test case

* Add test case

mypy_django_plugin/transformers/models.py

@@ -363,7 +363,7 @@ class AddRelatedManagers(ModelClassInitializer):
                     self.add_new_node_to_model_class(
                         attname, Instance(default_reverse_manager_info, []), no_serialize=True
                     )
-                    return
+                    continue
 
                 # The reverse manager we're looking for doesn't exist. So we create it.
                 # The (default) reverse manager type is built from a RelatedManager and the default manager on the related model