fix: only apply our pipeTo/pipeThrough optimisations to TransformStreams who have no transformers (IdentityStreams).

[JakeChampion]

We have a performance optimisation for when the receiver is a native source, whereby we append the entire source body to the destination using a single native hostcall, this greatly improves the performance (https://barstool.engineering/a-real-world-comparison-between-cloudflare-workers-and-fastly-compute-edge/)

This optimisation however, should only be applied when the TransformStream is being used in this particular, having not supplied a transformer to the TransformStream constuctor, i.e. an IdentityStream. If we apply the optimisation when a transformer is supplied, the transformer would never get called, which is not correct behavior.

We want to apply the optimisation in this scenario:

function combineStreams(streams: ReadableStream[]): ReadableStream {
  const stream = new TransformStream()
  _combineStreams(streams, stream.writable)
  return stream.readable
}

async function _combineStreams(sources: ReadableStream[], destination: WritableStream) {
  for (const stream of sources) {
    await stream.pipeTo(destination, {
      preventClose: true
    })
  }
  destination.close()
}

And we don't want to apply the optimisation in any other scenario such as:

function upperCaseStream() {
  return new TransformStream({
    transform(chunk, controller) {
      controller.enqueue(chunk.toUpperCase());
    },
  });
}

[triblondon]

👍 for the test case

[elliottt]

Looks great!