chan_sip: Trigger reinvite if the SDP answer is included in the SIP ACK

Under certain conditions, a SIP transaction involving directmedia wouldn't
trigger a re-invite because the SDP answer was included in an ACK instead
of in a message that we would have triggered the invite with. This patch
just queues a source change control frame if the dialog is using
directmedia when we find sdp for an ACK.

(closes issue AST-913)
Reported by: Thomas Arimont
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Merged revisions 371337 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 371338 from http://svn.asterisk.org/svn/asterisk/branches/10
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Merged revisions 371355 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@371356 65c4cc65-6c06-0410-ace0-fbb531ad65f3