[Fetch API] Allow used body replacement in Request constructor

[Harris Hancock]

Currently, the following code snippet replaces request's body as one would expect:

  let request = new Request(url, {
    method: "POST", body: "foo"
  })
  request = new Request(request, {
    body: "bar"
  })

But this snippet, in which the author tries to replace a used body, throws a TypeError:

  let request = new Request(url, {
    method: "POST", body: "foo"
  })
  await request.text()  // disturb the body
  request = new Request(request, {
    body: "bar"
  })  // throws

Since the second snippet is providing a new body to the constructor, there doesn't seem to be any technical need to throw.

I have a pull request in to change this in the Fetch spec here:

https://github.com/whatwg/fetch/pull/675

Accompanying WPT PR here:

https://github.com/w3c/web-platform-tests/pull/9931

[Radar WebKit Bug Importer]

<rdar://problem/49425609>

[Chris Dumez]

Created attachment 366323 [details]
Patch

[Chris Dumez]

ping review?

[youenn fablet]

Comment on attachment 366323 [details]
Patch

View in context: https://bugs.webkit.org/attachment.cgi?id=366323&action=review

> Source/WebCore/Modules/fetch/FetchRequest.cpp:241
> +            return Exception {TypeError, "Request input is disturbed or locked."_s };

s/{T/{ T/

[Chris Dumez]

Committed r243757: <https://trac.webkit.org/changeset/243757>