Fixed #4787, #5913 -- Updating the queryset on a `ModelChoiceField` or `ModelMultipleChoiceField` now updates its widget's choices.  The clean methods for `ModelChoiceField` and `ModelMultipleChoiceField` were changed to only allow choices in the specified queryset (instead of allowing all choices returned by the queryset model's default manager).


git-svn-id: http://code.djangoproject.com/svn/django/trunk@6670 bcc190cf-cafb-0310-a4f2-bffc1f526a37