Fix Attribute error when model inherit manager
Fix Attribute error when model inherit manager

AttributeError: 'NoneType' object has no attribute 'get_query_set'
When a Model inherit from an Abstract one which implement a Manager.
In this case '_default_manager' is defined but equal to None
 
iXioN/django-money,tsouvarev/django-money,AlexRiina/django-money,tsouvarev/django-money,iXioN/django-money,recklessromeo/django-money,recklessromeo/django-money,rescale/django-money