CRM-14708 accept POSTs without x_cust_id based on missing contact_id problem & existing logic making it optional

Conflicts:
	CRM/Core/Payment/AuthorizeNetIPN2.php
Merge pull request #3292 from eileenmcnaughton/CRM-14708

CRM-14708 accept POSTs without x_cust_id based on missing contact_id … …