[whatwg/fetch] Use case for Headers getAll (#973)

Since https://github.com/whatwg/fetch/commit/42464c8c3d2fd3437a19fc6afd2438a0fd42dde8 `Headers.prototype.getAll` has been deprecated/removed from the spec and implementations.

I understand that in browsers (including service workers) filtered responses don't include headers that could benefit from the getAll method.
However, some JavaScript environment doesn't need to filter response/request headers, like serverless plateforms (for instance Cloudflare Workers) or maybe Nodejs at some point.

For example editing Cookie headers:
```js
const h = new Headers;
h.append("Set-Cookie", "a=1; Expires=Wed, 21 Oct 2015 07:28:00 GMT");
h.append("Set-Cookie", "b=1; Expires=Wed, 21 Oct 2015 07:28:00 GMT");

h.get("Set-Cookie")
// a=1; Expires=Wed, 21 Oct 2015 07:28:00 GMT, b=1; Expires=Wed, 21 Oct 2015 07:28:00 GMT
```

Spliting the combined header value by `, ` will give an invalid result. Instead `getAll` could be used to retrieve the individual headers.

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