PJSIP: Handle headers in a list appropriately

The PJSIP header parsing function (pjsip_parse_hdr) can generate more
than one header instance from a single header field. These header
instances exist as a list attached to the returned header and must be
handled appropriately when they are added to a message or else only the
first header instance will be used. This changes the linked list
functions used in outbound proxy code to merge the lists properly.


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/12@406020 f38db490-d61c-443f-a65b-d21fe96a405b