chan_sip: trust_id_outbound CHANGES message improvement

(closes issue AST-1301)

(closes issue ASTERISK-19465)
Reported by: Krzysztof Chmielewski
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Merged revisions 412821 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 412822 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/12@412823 f38db490-d61c-443f-a65b-d21fe96a405b