[1.1.X] Fixed #11687: the `add` filter is now less failsome when faced with...

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

Backport of [12497] from trunk, although the fix here is slightly different to
avoid adding new behavior to a bugfix branch.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.1.X@12499 bcc190cf-cafb-0310-a4f2-bffc1f526a37