[whatwg/fetch] Allow detecting whether request method has changed (e.g. due to redirect) (#1212)

I propose to add a way for consumers to know whether the request method has changed.
This can happen, if a 301/302 redirect happened on a POST request.

Currently, there is only the `redirected` property, which includes 301/302 but also 307/308 which do not change the request method.

Unexpected things happen when the request method has changed, and there is currently no way to detect this.

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