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