Fixed #15499 -- Ensure that cache control headers don't try to set public and private as a result of multiple calls to patch_cache_control with different arguments. Thanks to AndiDog for the report and patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@16657 bcc190cf-cafb-0310-a4f2-bffc1f526a37