The SIP_HEADER function only works on the the initial SIP INVITE. The documentation was updated
in trunk, but not in 1.8 or 10, so I'm making them match.
(Closes issue ASTERISK-18640)
git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/1.8@338492 65c4cc65-6c06-0410-ace0-fbb531ad65f3