Update serializers.py

Treat the input queryset as it comes (maybe it has been changed in a higher level). Evaluating .all() does nothing if or if not is a queryset.
Possible we might be missing a test case for why this is needed?

When a serializer is applied as a child as eg. a many-to-many relationship, then the property will be a `RelatedObjectManager` (not a queryset) and I *think* we need `.all()` applying there.

Eg `a1.publications.all()` in the [examples here](https://docs.djangoproject.com/en/dev/topics/db/examples/many_to_many/).

Also possible that I'm being stupid and we don't actually need this at all.