Support ModelAdmin.preserve_filters

[smcoll]

Regarding this property (introduced in Django 1.6): https://docs.djangoproject.com/en/dev/ref/contrib/admin/#django.contrib.admin.ModelAdmin.preserve_filters

It seems that filter params are preserved through node creation until the final redirect to the changelist. The "actions" buttons do not support the filter params at all.

[vdboor]

Most of this went into django-polymorphic directly. If you still see issues with the latest versions, please let me know. For now I'll be closing this ticket.