Bugfix: N1QL Indexer.IndexByPrimary() method.

Change-Id: Ibe646c580a7e979b21f161368a386d7439ae9d1b
Reviewed-on: http://review.couchbase.org/44256
Reviewed-by: Sriram Melkote <siri@couchbase.com>
Tested-by: Sriram Melkote <siri@couchbase.com>