Added a test to ensure customer.subscriber fk points to the correct model
Added a test to ensure customer.subscriber fk points to the correct model

Added tests to ensure that the customer.subscriber fk always points to the model configured by DJSTRIPE_SUBSCRIBER_MODEL setting. And only when it is unset that the default fallback of AUTH_USER_MODEL is used.

pydanny/dj-stripe,dj-stripe/dj-stripe,pydanny/dj-stripe,dj-stripe/dj-stripe