Re: [fetch] Prevent modifying headers on Responses (#24)

Being able to add headers like this is kinda nice, but not essential. If we make them mutable, we should do the same with requests.

```js
self.addEventListener('fetch', function(event) {
  event.request.headers.append('foo', 'bar');
});
```

Of course, that shouldn't work with 'no-cors' requests, probably other kinds of requests too.

If editing headers on `fetchEvent.request`s & responses returned by fetch throws now, it's something we can open up later.

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