Fixed #159 -- Prevent the `AdminSite` from logging users out when they try to...

Fixed #159 -- Prevent the `AdminSite` from logging users out when they try to log in form the logout page. Many thanks, ashchristopher.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@17465 bcc190cf-cafb-0310-a4f2-bffc1f526a37