SQLAlchemyConnectionField sort works only for top-level query.

When I have SQLAlchemyConnectionField in top-level query, like in the [example](http://docs.graphene-python.org/projects/sqlalchemy/en/latest/tips/#sorting) it works perfectly.

But if I have `Query -> Viewer -> SomeObject -> PetConnection -> Pet`
sort won't work.
In case of top level query `UnsortedSQLAlchemyConnectionField.get_query` is called, which injects sorting.
In case of nested object connection resolve - gettattr already takes model object (SomeObject) with
Pets array populated from the db. So getattr just takes these pet without any ordering.

Workaround here is to have custom resolver, but it would be much better to fix it on library level.
Thanks!