String instead of binary was returned

GeometryCollection returned the property `geometries` as a string
and not as a binary.

Change-Id: I3d17afd97e92b1ec2b50ccc8cbde94a66a417d37
Reviewed-on: http://review.couchbase.org/41771
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Sarath Lakshman <sarath@couchbase.com>