[fetch] Allow operation to outlive the document unloading (beacon) (#124)

The [Beacon API](http://www.w3.org/TR/beacon/) adds a new feature to the platform — non-Service-Worker requests that outlive their document — but adds it in a high-level API with a narrow use-case.

There is no [lower-level primitive in that "explains"](https://extensiblewebmanifesto.org/) the Beacon API. Perhaps `fetch` could be it? 

```
function sendBeacon(url, data) {
    fetch(url, {
        method: 'post',
        body,
        detachedFromDocumentLifetime: !!"bikeshed",
        ageHeaderUpdatedOnRetry: 'Beacon-Age',
    });
}
```


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Reply to this email directly or view it on GitHub:
https://github.com/whatwg/fetch/issues/124