resolve the issue of ` update document must contain key beginning with '$' ` while bulk upsert operation.

when mongo driver is updated from go.mongodb.org/mongo-driver@v1.3.5 to go.mongodb.org/mongo-driver@v1.9.1 their is issue while bulkUpsert and bulkUpdate operation as update document must contain key beginning with '$' to resolve this need to add $set to model.SetUpdate.