Fix VM snapshot handling for combined INBOX.

The snapshot API contains an option that allow for combining of new 
and old messages within a single snapshot. New messages, however, 
include options beyond just 'INBOX' - it also includes the Urgent 
folder. A previous patch that combined INBOX and Urgent accidentally 
impacted snapshots that attempted to gain messages from just the Old 
folder. This patch fixes the snapshot gathering such that the API 
returns the appropriate messages for the folder selected, with and 
without the combine option.

This should make it more clear about what's happening.

Review: https://reviewboard.asterisk.org/r/2539/
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Merged revisions 388816 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@388818 65c4cc65-6c06-0410-ace0-fbb531ad65f3