MB-9330 Don't send unescaped JSON object keys

Ensure yajl preserves object keys' escaping, otherwise we
are using invalid JSON and sending it to the clients.

Change-Id: I5759943f33a299a572f9057fe13f45e26705fd0e
Reviewed-on: http://review.couchbase.org/29540
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Volker Mische <volker.mische@gmail.com>
Tested-by: Filipe David Borba Manana <fdmanana@gmail.com>