Re: [whatwg/fetch] Cloned responses should not have immutable headers (#608)

For my use case the Cache API.

Essentially what I'm wanting to do is something like this:

```js
const startTime = performance.now();
const response = await fetch(url);
const duration = performance.now() - startTime;
const wasCached = determineSomehowIfResponseCameFromCacheApi(response);

sendDataToAnalyticsService({
  url,
  duration,
  wasCached,
});
```

And since my service worker will sometimes be responding to the fetch event with a cached response, I want to know how often that happens and compare the actual savings via RUM.



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