Headers iterator throws when the response contains a Set-Cookie

[sbellone]

Bug Description

#1915 introduced a bug on Headers iterator, which now throws an error when the response contains a Set-Cookie header.

(Also, getSetCookie() returns an empty array for me)

Reproducible By

const express = require('express');
const undici = require('undici');

const app = express();
const port = 3000;
const URL = `http://localhost:${port}/`;

app.get('/', (req, res) => {
  res.setHeader('Set-Cookie', 'test=onetwo');
  res.send('Hello World!');
});
const serv = app.listen(port);

(async () => {
  const res = await undici.fetch(URL);
  // console.log(res.headers.getSetCookie()); // []

  for (const entry of res.headers.entries()) {
    console.log(entry);
  }

  serv.close();
})();

Expected Behavior

See arrays of key/values logged on the console (it works with Undici 5.18.0):

[ 'connection', 'keep-alive' ]
[ 'content-length', '12' ]
...

Logs & Screenshots

~/tests/node_modules/undici/lib/fetch/headers.js:430
        for (const value of cookies) {
                            ^

TypeError: cookies is not iterable
    at get [headers map sorted] (~/tests/node_modules/undici/lib/fetch/headers.js:430:29)
    at ~/tests/node_modules/undici/lib/fetch/headers.js:476:21
    at Headers Iterator.next (~/tests/node_modules/undici/lib/fetch/util.js:731:22)
    at ~/tests/fetch-undici-2.js:18:14
    at process.processTicksAndRejections (node:internal/process/task_queues:95:5)

Environment

Undici 5.19.1
Node v18.14.0