[whatwg/fetch] Fetch fails when using keepalive option with headers (#924)

When sending a POST request with `fetch` that contains headers and the `keepalive` option enabled, the request fails, returning the error: "Failed to fetch."

I did not see anything in the spec that would preclude the use of custom headers with `keepalive`. 

In the following code, the first `fetch` returns a response, whereas the second one throws the error mentioned above. It exhibits the same behavior with both standard and custom headers.

[Link to the code on CodePen](https://codepen.io/ObjectiveCat/pen/aboWEoa)

```javascript
(async () => {
  
  const URL = 'https://jsonplaceholder.typicode.com/posts';
  const body = JSON.stringify({data: 'test'});
  
  // keepalive flag 
  
  const fetchOptions = {
    method: 'POST',
    keepalive: true,
    body
  }
  
   try {
    const result = await fetch(URL, fetchOptions);
    console.log(await result.json());
  } catch(e) {
    console.error(`Fetch failed: ${e}`);
  }
  
  
  // keepalive flag with headers:
  
  const fetchOptionsHeaders = {
    ...fetchOptions,
    headers: {
      'Content-Type': 'application/json'
    }  
  };
 
  try {
    const resultHeaders = await fetch(URL, fetchOptionsHeaders);
    console.log(await resultHeaders.json()); 
  } catch(e) {
    console.error(`Fetch with headers failed: ${e}`);
  }

})();
```

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