The BatchSize change stream option is passed into $changestream aggregation pipeline instead to the cursor of the aggregation within Watch().

When performing:

	changeStreamOpts := changestreamopt.BundleChangeStream(changestreamopt.BatchSize(1000))
	cursor, err := collection.Watch(context.Background(), nil, changeStreamOpts)

Resulting in error message:

2018/10/08 14:53:47 (Location40415) BSON field '$changeStream.batchSize' is an unknown field.

Looking at the code
https://github.com/mongodb/mongo-go-driver/blob/v0.0.15/mongo/change_stream.go#L66-L73

only maxAwaitTimeMS is passed to the cursor, but batchSize is passed into the pipeline operator.

This affects current version 0.0.15