chan_sip: trust_id_outbound CHANGES message improvement

(closes issue AST-1301)

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


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/1.8@412821 f38db490-d61c-443f-a65b-d21fe96a405b