[whatwg/streams] Default transform() method should be the identity transform (#565)

Currently `transformer.transform()` is compulsory. The TransformStream constructor will throw a TypeError if you do not set it.

I think instead if `transformer.transform` is undefined it should be equivalent to

```javascript
transform(chunk, controller) {
   controller.enqueue(chunk);
}
```

This enables two useful things:
* A transformer which just adds a fixed header and footer can be implemented with just `start()` and `flush()`.
* A transformer which just buffers can by implemented by specifying an empty _transformer_ and a strategy.

Both these things can be done by specifying the `transform()` method explicitly, but skipping it

1. Is shorter,
2. Enables optimisations by making the transformation unobservable from Javascript.

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