[Reviewer Matt] Don't print existing Content-Length header if adding a new one

Matt

Can you review this fix for me - PJSIP was adding extra Content-Length headers when transmitting messages, so after a sequence of forwarding a message to various places we could end up with several Content-Length headers.  This fixes sprout issue https://github.com/Metaswitch/sprout/issues/706.

I've tested by running some UTs and manually inspecting the results.

Mike