Fixed #7776: Ensured that the test cookie is always deleted once a login has...

Fixed #7776: Ensured that the test cookie is always deleted once a login has succeeded. Thanks for the report and fix, Mnewman.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@8484 bcc190cf-cafb-0310-a4f2-bffc1f526a37