Re: [fetch] Aborting a fetch (#27)

@appden 

```js
const responsePromise = fetch(url).then(parseResponse);
const resultPromise = responsePromise.then(retrieveResult);
```

`responsePromise.cancel()` would also result in the cancelation of `resultPromise` if `responsePromise` has not yet settled (meaning `resultPromise` has not yet settled).

`resultPromise.cancel()` would also result in the cancelation of `responsePromise` if `resultPromise` has not setted *and* `responsePromise` has not settled.

```js
const responsePromise = fetch(url).then(parseResponse);
const resultPromise = responsePromise.then(retrieveResult);
const anotherResultPromise = responsePromise.then(retrieveResultDifferently);
```

Here, canceling `anotherResultPromise` will only cancel `anotherResultPromise`. `responsePromise` is not canceled because `resultPromise` is interested in the result.

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