Prevent conflicts regarding unsolicited and solicited MWI to an endpoint.
If an endpoint is receiving unsolicited MWI for a mailbox and then attempts
to subscribe to an AOR that provides MWI for the same mailbox, then the SUBSCRIBE
is rejected with a 500 response.
Review: https://reviewboard.asterisk.org/r/3345
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Merged revisions 410590 from http://svn.asterisk.org/svn/asterisk/branches/12
git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@410591
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