[Reviewer Alex] Fixes issue #847 - don't generate a GRUU if the UE doesn't support them

Alex

As discussed, can you review my fix to only generate a GRUU if the UE claims to support them (issue #847).  I've tested by leaving GRUU supported turned on in most of the RegistrarTest cases, and adding a new test case that turns it off and checks the pub-gruu is omitted from the Contact header in the REGISTER response.

Mike