[whatwg/streams] piping issue (#509)

I have included the [web streams polyfill](https://github.com/creatorrr/web-streams-polyfill) into my project and i'm trying to pipe a readableStream to a writableStream 

This is very simplified but it seems to work fine (it dose what i want it to do).
The writable stream gets all chunks it closes when the readableStream closes.
```javascript
var rs = new ReadableStream({
 start(c) {
  c.enqueue(5);
  c.close();
 }
})

var ws = new WritableStream({})

rs.pipeTo(ws)
```
But as soon as i `.close()` the ReadableStream it logs 3 TypeError

```
Uncaught (in promise) TypeError: Reader was released and can no longer be used to monitor the stream's closedness(…)
Uncaught (in promise) TypeError: Writer was released and can no longer be used to monitor the stream's closedness(…)
Uncaught (in promise) TypeError: Writer was released and can no longer be used to monitor the stream's closedness(…)
```
I'm doing something wrong?

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