Failing test for nested object deletion

While working on one-to-many writable serializers I came across this bug.  Basically we don't pass the `._deleted` attribute up to the parent serializer (the only one which calls save).  I'm not sure the best approach to fix this.  I think we should fix it before opening the one-to-many pull request though.

Also, I wasn't sure the best place to put the test.  I needed a combination `serializer_bulk_update.py` and `serializer_nested.py` test setup.

Thoughts?