Make Source.customer nullable
Make Source.customer nullable

When a source is detached, the customer is set to None.

pydanny/dj-stripe,kavdev/dj-stripe,jleclanche/dj-stripe,pydanny/dj-stripe,dj-stripe/dj-stripe,jleclanche/dj-stripe,dj-stripe/dj-stripe,kavdev/dj-stripe