apps/app_voicemail: Fix IMAP header compatibility issue with Microsoft Exchange

When interfacing with Microsoft Exchange, custom headers will be returned as
all lower case. Currently, the IMAP header code will fail to parse the returned
custom headers, as it will be performing a case sensitive comparison. This can
cause playback of messages to fail, as needed information - such as origtime -
will not be present.

This patch updates app_voicemail's header parsing code to perform a case
insensitive lookup for the requested custom headers. Since the headers are
specific to Asterisk, e.g., 'x-asterisk-vm-orig-time', and headers should be
unique in an IMAP message, this should cause no issues with other systems.

ASTERISK-24787 #close
Reported by: Graham Barnett
patches:
  app_voicemail.c.patch_MSExchange uploaded by Graham Barnett (License 6685)
........

Merged revisions 432012 from http://svn.asterisk.org/svn/asterisk/branches/11
........

Merged revisions 432013 from http://svn.asterisk.org/svn/asterisk/branches/13


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@432014 65c4cc65-6c06-0410-ace0-fbb531ad65f3