Fixed #8049 -- Fixed inconsistency in admin site is_active checks. Thanks for...

Fixed #8049 -- Fixed inconsistency in admin site is_active checks. Thanks for patch and tests, isagalaev

git-svn-id: http://code.djangoproject.com/svn/django/trunk@12159 bcc190cf-cafb-0310-a4f2-bffc1f526a37