Applicator.get_basket_offers() checks for applicable basket offers using
something like if voucher.is_available_to_user(user) (see
https://github.com/django-oscar/django-oscar/blob/master/src/oscar/apps/offer/utils.py#L99).
However, the return value of is_available_to_user() is a 2-tuple, so
this will always evaluate to True.

Closes #1647