chan_sip: trust_id_outbound CHANGES message improvement

(closes issue AST-1301)

(closes issue ASTERISK-19465)
Reported by: Krzysztof Chmielewski
........

Merged revisions 412821 from http://svn.asterisk.org/svn/asterisk/branches/1.8


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/11@412822 65c4cc65-6c06-0410-ace0-fbb531ad65f3