Normalize headers in interceptors

This would solve...

Some interceptors assume the request headers are necessarily objects: https://github.com/nodejs/undici/blob/6551919a3e492746806604cec2d241385c391f35/lib/interceptor/cache.js#L238-L242 https://github.com/nodejs/undici/blob/6551919a3e492746806604cec2d241385c391f35/lib/interceptor/dns.js#L360-L363 This creates problems when headers are not objects or when they get converted to an array, like in the redirect interceptor: https://github.com/nodejs/undici/blob/6551919a3e492746806604cec2d241385c391f35/lib/handler/redirect-handler.js#L231-L236 Using the redirect handler with the dns one, in fact, will completely break headers (see example below)

The implementation should look like...

IMHO, the interceptors should convert the request headers to an object before using them, considering that it makes them easy to manipulate and most users will provide them already as objects, so no conversion is needed.

I have also considered...

An alternative could be to fix the interceptors to handle the headers based on the possible types, but this would make very difficult to implement some logics, like checking the value of some headers (which is needed in the cache interceptor)

Additional context

Using redirect and dns interceptors:

createServer((req, res) => {
	if (req.url?.endsWith("/redirect")) {
		console.log(req.headers);
		exit();
	}
	res.setHeader("location", "/redirect");
	res.statusCode = 302;
	res.end();
}).listen(8008);

request("http://localhost:8008", {
	headers: { "user-agent": "something", hello: "world" },
	dispatcher: new Agent().compose(
		interceptors.dns(),
		interceptors.redirect({ maxRedirections: 1 }),
	),
});
// This logs
// {
//   '0': 'user-agent',
//   '1': 'something',
//   '2': 'hello',
//   '3': 'world',
//   host: 'localhost',
//   connection: 'keep-alive'
// }

Read the header handling around the referenced lines in lib/interceptor/cache.js, lib/interceptor/dns.js, and lib/handler/redirect-handler.js. Reproduce the redirect-plus-DNS example first, then verify that the combined flow preserves request headers in a usable object form without numeric entries.