res_pjsip_session: Be less strict with core requested outgoing capabilities.

res_pjsip_session: Be less strict with core requested outgoing capabilities.

The core may (depending on circumstances) request a single codec on outgoing
calls. Many channel drivers ignore or treat this as a suggestion while still
including configured codecs. The res_pjsip_session logic treated this as
an explicit request, leaving out other configured codecs.

This change makes res_pjsip_session behave like other channel driver and simply
adds the requested codec to the list.

(closes issue ASTERISK-23082)
Reported by: xrobau

Review: https://reviewboard.asterisk.org/r/3140/
........

Merged revisions 406489 from http://svn.asterisk.org/svn/asterisk/branches/12

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@406490 65c4cc65-6c06-0410-ace0-fbb531ad65f3