use redirect_to_login instead of HttpResponseRedirect in get_403_or_None

Using redirect_to_login facilitates correct interpretation of settings.LOGIN_URL when it is a class (views.login) instead of path (/login). This matches functionality in django-braces. Without this change the redirect will send you to the wrong page ie: /views.login/?next=/delete/1/.
This would leave `LOGIN_URL` with changed value for other tests (configuration leakage).

Please use `guardian.testapp.tests.conf.override_settings` (backported to work for older Django versions; see  https://docs.djangoproject.com/en/dev/topics/testing/tools/#django.test.override_settings)