chan_sip: Do not increment the SDP version between 183 and 200 responses.

chan_sip: Do not increment the SDP version between 183 and 200 responses.

Bumping the SDP version number can cause interoperability problems
since receivers of the responses will expect that a 200 SDP will
be identical to a previous 183 SDP.

(closes issue ASTERISK-21204)
reported by NITESH BANSAL

Patches:
	dont-increment-session-version-in-2xx-after-183.patch uploaded by NITESH BANSAL (License #6418)
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Merged revisions 400906 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 400908 from http://svn.asterisk.org/svn/asterisk/branches/11

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/12@400910 65c4cc65-6c06-0410-ace0-fbb531ad65f3