Fix reporting _id for Bulk API edge case for pre-2.6 servers 3.0.x port

When upserted _id is not returned on custom _id's
The only issue I see here is that the BsonDocumentWrapper will decode the entire replacement document just to get the id.  But as this is already going to be woefully slow (bulk api against 2.4) I think it's ok to leave it as is.