Fixed #12850 -- Ensured that constraint names are quoted when they are...

Fixed #12850 -- Ensured that constraint names are quoted when they are removed. Thanks to Tim Kersten for the report and patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@12811 bcc190cf-cafb-0310-a4f2-bffc1f526a37