[Reviewer Matt] Fix to issue 523 (DefaultHandling processing for non-REGISTER requests)

Matt

Can you review my fix to issue 523.  This also fixes issue 506.  I've also cleaned the code up a bit by using an enum (rather than a booléan) and using the enum consistently between the non-REGISTER IFC handling and third-party REGISTER handling.  I've tested by updating and adding UTs as appropriate.

Mike