github email testcase

test.txt

Interesting bug, stripe needs to be able to distinguish between setting an
attribute to None which exists in the saved entity and setting an attribute
to None which hasn't been saved yet.

A hack to fix this would be to manually remove "coupon" from
customer._unsaved_values.

It looks like the __setattr__ method could be modified to check whether the
key is in _retrieve_params and if it isn't and the value is None then
remove the key from _unsaved_values. I'll put together a pull request when
I get back later.