[1.2.X] Fixed #11377: the template join filter now correctly escapes the joiner, too.

Thanks, Stephen Kelly.

Backport of [13464] from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@13465 bcc190cf-cafb-0310-a4f2-bffc1f526a37