Used get_username() instead of referencing the username attribute

Since the User model can be swapped out, [this method](https://docs.djangoproject.com/en/1.6/ref/contrib/auth/#django.contrib.auth.models.User.get_username) should be use instead of referencing the username attribute directly.

Unfortunately I have not the time to investigate more but you probably shouldn't assume a `username` in tests e.g.:

- https://github.com/tangentlabs/django-oscar/blob/d30fc8cb0a1d81f59b97c8f4ef2d651629064f6d/oscar/test/testcases.py#L32
- https://github.com/tangentlabs/django-oscar/blob/20df512edac56a97539251c6f5ede9046273255e/tests/_site/myauth/models.py#L25
- https://github.com/tangentlabs/django-oscar/blob/d30fc8cb0a1d81f59b97c8f4ef2d651629064f6d/tests/functional/customer/history_tests.py#L53