chan_sip: Add MESSAGE request to allowed methods

The allowed methods advertised by chan_sip did not previously note the MESSAGE
request. Even in Asterisk 1.8, we do accept in-dialog MESSAGE requests; we
should advertise that we support MESSAGE requests.

ASTERISK-23504 #close
ASTERISK-23504 #comment Reported by: Martin Kontsek
ASTERISK-23504 #comment Patch sip.h_patch.diff uploaded by Martin Kontsek (license 6587)

Review: https://reviewboard.asterisk.org/r/3396/
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Merged revisions 411372 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 411373 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/12@411374 f38db490-d61c-443f-a65b-d21fe96a405b