Prevent infinite recursion in Django 1.6

Using the polymorphic admin with Django 1.6 yields the following error for me:

    ...
    File "/home/vagrant/virtualenv/lib/python2.6/site-packages/polymorphic/admin.py" in get_form
      412.         return super(PolymorphicChildModelAdmin, self).get_form(request, obj, **kwargs)
    File "/home/vagrant/virtualenv/lib/python2.6/site-packages/django/contrib/admin/options.py" in get_form
      503.             fields = flatten_fieldsets(self.get_fieldsets(request, obj))
    File "/home/vagrant/virtualenv/lib/python2.6/site-packages/polymorphic/admin.py" in get_fieldsets
      478.         other_fields = self.get_subclass_fields(request, obj)
    File "/home/vagrant/virtualenv/lib/python2.6/site-packages/polymorphic/admin.py" in get_subclass_fields
      497.         form = self.get_form(request, obj, exclude=exclude)
    File "/home/vagrant/virtualenv/lib/python2.6/site-packages/polymorphic/admin.py" in get_form
      412.         return super(PolymorphicChildModelAdmin, self).get_form(request, obj, **kwargs)
    File "/home/vagrant/virtualenv/lib/python2.6/site-packages/django/contrib/admin/options.py" in get_form
      503.             fields = flatten_fieldsets(self.get_fieldsets(request, obj))
    File "/home/vagrant/virtualenv/lib/python2.6/site-packages/polymorphic/admin.py" in get_fieldsets
      478.         other_fields = self.get_subclass_fields(request, obj)
    File "/home/vagrant/virtualenv/lib/python2.6/site-packages/polymorphic/admin.py" in get_subclass_fields
      497.         form = self.get_form(request, obj, exclude=exclude)
    File "/home/vagrant/virtualenv/lib/python2.6/site-packages/polymorphic/admin.py" in get_form
      412.         return super(PolymorphicChildModelAdmin, self).get_form(request, obj, **kwargs)

    Exception Type: RuntimeError at /admin/fluent_pages/page/3/
    Exception Value: maximum recursion depth exceeded in cmp

This is due to the changes in the way the django `get_form` function handles `fields`. In 1.5 it simply inspected `self.declared_fieldsets` but in 1.6 it now calls `self.get_fieldsets()`, which results in recursion.