Fix model subclass ___ selector for abstract/proxy models

[vdboor]

As discovered in django-fluent/django-fluent-pages#25, django-polymorphic has a bug in translate_polymorphic_field_path with traversing abstract/proxy models. Reporting this issue here as reminder for ourselves (and anyone with time to look into it before we do ;)

[mrmachine]

It would be great to be able to filter child models via proxy models. I want to add some extra queryset methods to a proxy model, but it seems that won't be possible as it stands.