[Reviewer: Mike] Check P-Asserted-Identity header when determining whether caller is E164

Mike,

Please can you review this fix to issue #223?  I've changed the logic to look at the P-Asserted-Identity header first, and the From header only if there is no P-Asserted-Identity (which there should always be).  I've also fleshed out the UT - this wasn't covered at all before!

Matt