🐛 Bug Report — fetch not checking if resolved redirect URL is http(s)

Reproduction:

```
export default {
async fetch(req, env) {
if (req.url.endsWith('/sub')) {
return new Response("Hello World\n", {
status: 302, headers: {
location: 'file://what/is/the/url'
}
});
} else {
const resp = await fetch('http://localhost:8080/sub', { redirect: 'follow'});
console.log(resp.status);
console.log(resp.url);
return new Response("ok");
}
}
};
```

Expected result:

A useful error or proper network error response as defined by the spec

Actual result:

The runtime throws an "internal error" due to a triggered assert in `kj/compat/http.c++`

What's the issue:

In the initial request, the runtime will validate that the request URL is `http://` or `https://`, with additional special handling around `ws://` and `wss://` URLs. When a redirect response is processed and we generate a new request URL from the `location` header, we are not applying the same checks so we end up passing a non-http URL down to the kj http client, which fails with an assertion. We need to be properly validating the redirect url.