Fix draft 3 'required' validation issues

This request adds 2 sets of failing draft3-'required' testcases and the resp. fixes

Testcases:

 * commit 8d5443eccc9c5427f048556dee54568abda64f73: draft3 'required' constraints which are satisfied produce php warnings ( if the required property is given by an object instance).

 * commit e89b755aab01dd9097079f8cf922d896f232db61: draft3 'required' properties which are given by object instances are not type checked.