Extract ObjectId from DBRef arrays

Since issue #50 was closed with commit 8043b96, I'd like to try contributing.

When an Array contains DBRef objects, it's best to use the $oid BSON::ObjectId  string value instead of the default string representation of DBRef which is not a valid hash. This is consistent with the conversion made for single DBRef values. This has been tested with both JSON and TEXT ARRAY columns types.