Fix draft 3 'required' validation issues

[alexmmm]

This request adds 2 sets of failing draft3-'required' testcases and the resp. fixes

Testcases:

• commit 8d5443e: draft3 'required' constraints which are satisfied produce php warnings ( if the required property is given by an object instance).
• commit e89b755: draft3 'required' properties which are given by object instances are not type checked.