Fixed #7416: Modified test client to preserve session when a user logs in....

Fixed #7416: Modified test client to preserve session when a user logs in. Thanks to lakin.wecker@gmail.com for the report and Eric Holscher for the patch and test case.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@8372 bcc190cf-cafb-0310-a4f2-bffc1f526a37