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

I'm not qualified to comment on if it is appropriate or not to do something like:

```js
// serviceworker.js
onfetch = function(e) {
  e.respondWith(fetch(e.request).then(function(response) {
    response.headers.append("Reply-from-SW", "yes");
    return response;
  })
}
```

but from talking with @annevk on IRC, it seems this is not ok.

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https://github.com/whatwg/fetch/issues/24