Return (smi) 0 instead of object null from the FILTER_KEY builtin.

Add a test which tests keys being deleted during for-in.
Review URL: http://codereview.chromium.org/3170004

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@5243 ce2b1a6d-e550-0410-aec6-3dcde31c8c00