Added a failing testcase.

Hi there!

I've been watching your project for a couple of weeks since it simplifies a lot of things in a code base of mine but I hit a wall when trying to use it in an inheritance scenario.

As described in the provided testcase I'm trying to reverse through a parent relationship but it looks like the whole query generation gets confused. Apart from the missing field exception I noticed that an `INNER JOIN` is used instead of a `LEFT OUTER` one.

I'm trying to familiarize myself with the Django ORM therefore an hint as where to start looking would be greatly appreciated.