PCBC-227: Allow store of pure numeric keys

This does however break the test for PCBC-191, because the index in
the array will be used as the key. Is that intentional?

Change-Id: Ie52ffad1cea2b996ed66762fd816df6039df2741
Reviewed-on: http://review.couchbase.org/26109
Tested-by: Mordechai Nunberg <mnunberg@haskalah.org>
Reviewed-by: Matt Ingenthron <matt@couchbase.com>