[1.2.X] Fixed #13283 -- Corrected CACHE_MIDDLEWARE_ANONYMOUS_ONLY's bad habit...

[1.2.X] Fixed #13283 -- Corrected CACHE_MIDDLEWARE_ANONYMOUS_ONLY's bad habit of setting Vary: Cookie on all responses and destroying cache efficiency. Thanks to natrius for the fix.

Backport of r15381 from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@15382 bcc190cf-cafb-0310-a4f2-bffc1f526a37