Ruby 777

With pre-2.6 servers, the bulk API has an edge case around documents with non-ObjectID _id fields.  This pull request offers a workaround for the issue.
'batch' is an array of specific operations that happened as a part of this single 'update' parent operation.  Since it's an upsert, there will only ever be one batched operation by definition, which is why I've assumed it is at index 0.  At least, that's how I understand it :)