Fixed #11687: the `add` filter is now less failsome when faced with things...

Fixed #11687: the `add` filter is now less failsome when faced with things that can't be coerced to integers.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@12497 bcc190cf-cafb-0310-a4f2-bffc1f526a37