Fix default handling for non-REGISTER messages
Bring default handling for REGISTERs in to line with non-REGISTER case (so we either break none or break both!)
Add UTs for missing and malformed DefaultHandling attribute in IFCs
Code review feedback
Merge pull request #528 from Metaswitch/iss523

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