[1.1.X] Fixed #13152 -- Ensure the test client saves the session before...

[1.1.X] Fixed #13152 -- Ensure the test client saves the session before writing the session key to the cookie, in case the session engine changes the session key.

Backport of r12806 from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.1.X@12807 bcc190cf-cafb-0310-a4f2-bffc1f526a37