[fetch] Request constructor does not check used flag on null body (#55)

Over in https://github.com/yutakahirano/fetch-with-streams/issues/37#issuecomment-103587329 @yutakahirano noticed that this code may not do what is expected:

```
var req = new Request(url, {body: 'hello'});
fetch(req);
fetch(req);
```

I think we would expect the second fetch() to reject on a drained Request, but it won't.  The first fetch() sets the request's used flag to true and its body to null.  The second fetch(), however, only looks at the used flag if the body is not null.

What do you think @annevk?

---
Reply to this email directly or view it on GitHub:
https://github.com/whatwg/fetch/issues/55