correctly parse isROAdmin in _cbauth response

Change-Id: If3730cb21ed5bcb6ffdeac18646c7362635f694c
Reviewed-on: http://review.couchbase.org/44546
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>