Improve fix of PR #321
Improve fix of PR #321

checking hasattr(self, 'fieldsets') alway returns true, and produces
different effects. The hasattr(self, 'declared_fieldsets') was designed
to detect whether Django injected that attribute.

Amends: 5e150ea85ab7821adf9bdb730979ef89546ecc0a

chrisglass/django_polymorphic,chrisglass/django_polymorphic