[Reviewer: Ellie] Subscription headers

Fixes https://github.com/Metaswitch/sprout/issues/505. Tested in UT, but not live.

I think that, previously, the "Retry-After: 0" header added at https://github.com/Metaswitch/sprout/blob/dev/sprout/stack.cpp#L370 wouldn't have gone into the message (because PJUtils::respond_statelessly ignored lists only one header long) - I've now fixed that bug.

Pass back to me rather than merging - I need to check one point of PJSIP memory pool arcana with Mike.