Convert an isinstance() back to a type()

This reverts one line of 673e5dd.  Previously it was checking if the type() of
something was QuerySet (and erroring if so).  The change caused it to error on
*any* instance of QuerySet, including for example a ValuesListQuerySet.  This broke some (or all?) ajax autocompletes.

It may be worth a more careful look over #1411 to make sure no other similar issues were introduced.
Perhaps this should be ``if not isinstance(query_set, ValuesQuerySet)``. Seems weird to only fail if a ``QuerySet`` is returned.

Also, this could use a comment, saying why this line is here. Seems it is here only because someone might make a ``ajax_autocomplete`` method that returns a ``QuerySet`` instead of a ``ValuesQuerySet``, and we assume the latter in the code below.

Better yet, this can be documented in the docstring. ``request.GET.get('ajax_func', 'ajax_autocomplete')`` must refer to a method on ``Model`` or ``Model.objects`` which returns a ``ValuesQuerySet``.