MB-11137 Fix sending mail alerts for compound keys.

Change-Id: I48ac4f7cf3be063af06d3ace20eee5635bca9161
Reviewed-on: http://review.couchbase.org/37207
Tested-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>