Fix FormFields w/ queryset but no limit_choices_to

Django assumes a form field with a `queryset` attribute always has a `limit_choices_to` attribute.
For example, a field programmatically altered to have a `queryset` attribute (which works fine), may not be altered to also have a `limit_choices_to` attribute. So the assumption and code are wrong.
Fix proposed for master, affects 1.7 and above.