Fixed #13092 -- Added support for the "in" operator when dealing with context...

Fixed #13092 -- Added support for the "in" operator when dealing with context lists. Thanks to clelland for the patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@13510 bcc190cf-cafb-0310-a4f2-bffc1f526a37