[whatwg/fetch] Fix body handling on the Request constructor (#458)

As Request.body is exposed, we need to specify how to handle the body property of the input Request more correctly. After

```
let r = new Request(request, ...);
```

successfully returns,

 - `request.body` should remain unchanged.
 - `request.bodyUsed` should be true.

Fixing https://github.com/yutakahirano/fetch-with-streams/issues/60.
You can view, comment on, or merge this pull request online at:

  https://github.com/whatwg/fetch/pull/458

-- Commit Summary --

  * initial
  * fix

-- File Changes --

    M fetch.bs (59)

-- Patch Links --

https://github.com/whatwg/fetch/pull/458.patch
https://github.com/whatwg/fetch/pull/458.diff

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