[Reviewer Keith] Fixes #54 - correctly find next route header

Keith

Can you take a quick look at this - found it while testing my authentication changes.  I think it was benign in the sense that tests would run, but non-INVITE requests would spiral through Bono a couple of extra times.

I'll take an action to see if we can get some extra coverage in edge proxy UT to catch any breakages here in future.

Mike