[1.2.X] Corrected an issue which could allow attackers to manipulate session...

[1.2.X] Corrected an issue which could allow attackers to manipulate session data using the cache. A security announcement will be made shortly.

Backport of r16759 from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@16765 bcc190cf-cafb-0310-a4f2-bffc1f526a37