[Reviewer: Andy] Preserve quotes when parsing Accept/Reject-Contact headers, but strip th...

...em when doing comparisons

Also includes custom headers tests to check this, and make sure all the cloning is valid.

Fixes https://github.com/Metaswitch/pjsip-upstream/issues/26.
As discussed, the re-ordering puts ;require;explicit at the end (not the start), which is the pattern generally used in the specs.