The :type of field use like foreign_key is String by default

If you use only BSON::ObjectID, when you define an association with references_many, The foreign_key is create without :type. So in Mongoid, the default type is String. But we want this foreign_key in same type of primary key.

I made a path to do that.

http://github.com/shingara/mongoid/commit/9d743b165ca348e023aefdd6161bbb5a2bf6697d