Description

The cache interceptor's normalizeHeaders function only handled two header
formats:

1. Array-of-pairs: [['key', 'val'], ...]
2. Object: { key: 'val', ... }

However, undici internally uses flat alternating arrays:
['key', 'val', 'key', 'val', ...]. When the cache() interceptor is placed
before dns() in a compose chain (e.g., new Agent().compose(cache(), dns())),
the DNS interceptor converts headers to flat format before passing them to
the cache interceptor, causing normalizeHeaders to throw
'opts.headers is not a valid header map'.

Reproduction

import { Agent, interceptors, request } from "undici";

const { cache, dns } = interceptors;
const cacheThenDns = new Agent().compose(cache(), dns());
const url = "https://www.google.com";
const headers = [];
const r = await request(url, { dispatcher: cacheThenDns, headers });

Fixes #5522