There was a bug that ignored `Request` options when one was given to the `fetch` function:
```ts
const request = new Request("https://example.vercel.sh", { method: "POST" });
await fetch(request);
```
The code above was expected to make a `POST` request, but instead it
made a `GET` request.
This commit fixes it and adds some tests to verify that fetching with a
`Request` object works as expected, and therefore resolves#37123.
## Bug
- [x] Related issues linked using `fixes #number`
- [x] Integration tests added
- [ ] Errors have helpful link attached, see `contributing.md`
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