[fetch] Request body should be removed when method changed to GET (#187)

https://fetch.spec.whatwg.org/#http-fetch 5.redirect.10.4

> 4. If either actualResponse's status is 301 or 302 and request's method is `POST`, or actualResponse's status is 303, set request's method to `GET`.

The body should be removed at this point to avoid a `GET` request with a body.

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Reply to this email directly or view it on GitHub:
https://github.com/whatwg/fetch/issues/187