[Reviewer Ellie] Correctly format and parse Path headers.

Ellie

Can you review my fix to https://github.com/Metaswitch/sprout/issues/743.  I've tested by updating registrar and edge proxy test cases, writing a custom_header test case, and running the live tests.

Note that I also fixed a problem I noticed with the Service-Route header parser I added yesterday - according to the RFC, Service-Route can have header parameters, but the parser wasn't handling this.

In terms of compatibility, the old Sprout would accept Path headers with or without <>, and would reflect whatever it got in the 200 OK response.  With this change, it will now only accept Path headers with <>, so we must be sure to upgrade Bono nodes first.  Perimeta compatibility is unaffected because it already uses <>.

Mike