- Compatibility with django-polymorphic
- Compatibility with django-polymorphic

django-polymorphic self._get_pk_value returns the class object.  If
this happens, we try adding _id to the pk_field.

johnrtipton/django-auditlog,Zmeylol/auditlog,rauleb/django-auditlog,robmagee/django-auditlog,chris-griffin/django-auditlog,jjkester/django-auditlog,kbussell/django-auditlog