Fix as_of manager method on class
Fix as_of manager method on class

treyhunner/django-simple-history,luzfcb/django-simple-history,emergence/django-simple-history,treyhunner/django-simple-history,pombredanne/django-simple-history,luzfcb/django-simple-history,pombredanne/django-simple-history,emergence/django-simple-history