fix(ext/fetch) Fix request clone error in flash server

[tsar-boomba]

Fixes #15858

Request cloning was not properly handling the flash inners.

Code like this now works as expected.

import { assertEquals } from 'https://deno.land/std@0.158.0/testing/asserts.ts';

// POST, PUT, and PATCH panic deno
Deno.serve(async (req) => {
	const cloned = req.clone();
	assertEquals(cloned.headers, req.headers);
	console.log(await req.text(), await cloned.json());
	return new Response('ok', {
		status: 200,
	});
});

[crowlKats]

LGTM