Do not blindly override select_related or prefetch_related.

When empty `select_related` or `prefetch_related` attributes are given
to subclasses of `SelectRelatedMixin` and `PrefetchRelatedMixin`, the
original query set should be kept: it may already have its own
select_related or prefetch_related.
Merge pull request #149 from Polyconseil/fix/dont_override_select_related_from_queryset

Do not blindly override select_related or prefetch_related.