Properly restore BSON::ObjectId from ES document using :model wrapper.

When you use :model wrapper, and it loads object attributes from ES to Mongoid model, it sets your association's id as {"$oid" => BSON::ObjectId()}, where it should just be BSON::ObjectId().

Some related info
https://github.com/mongoid/mongoid/pull/2947

https://github.com/rs-pro/mongoid-elasticsearch/issues/1