Content-Encoding header not updated appropriately

Repro:

```js
export default {
async fetch(req, env) {

const resp = await fetch('http://www.cloudflare.com', {
headers: {
'accept-encoding': 'gzip'
}
});

const headers = new Headers();
headers.set('x-content-encoding', resp.headers.get('content-encoding'));

let length = 0;
const dec = new TextDecoder();
for await (const chunk of resp.body) {
length += chunk.length;
console.log(dec.decode(chunk, { stream: true }) );
}
console.log(dec.decode() );
headers.set('x-decompressed-length', length);

return new Response("Hello World\n", { headers } );
}
};
```

When workerd decompresses the body, it still reports the `content-type` as being `gzip` when it should be removing the coding from the header.

In contrast, if `accept-encoding` is changed to `br`, the data read is still compressed and the `content-encoding` header is still `br`...

Which means users cannot rely on the `content-encoding` header as signal, the data still may or may still be encoded.

this is not an issue in production. the faulty handling here is within workerd.