[fetch] 5.1.0 regressed ability to intercept headers

[panva]

Bug Description

Intercepting upon individual headers when using fetch is broken in 5.1.0, worked fine in 5.0.0

#1309 and possibly lack of test coverage for this is imho the likely suspect

Reproducible By

import * as undici from 'undici'
import * as assert from 'node:assert'

const mockAgent = new undici.MockAgent()
mockAgent.disableNetConnect()
undici.setGlobalDispatcher(mockAgent)
const pool = mockAgent.get('https://example.com')
pool.intercept({
  path: '/foo',
  method: 'GET',
  headers: {
    accept: 'application/json'
  }
}).reply(200, { ok: 1 })

const response = await undici.fetch('https://example.com/foo', {
  headers: {
    accept: 'application/json'
  }
})

assert.deepStrictEqual(await response.json(), { ok: 1 })

Expected Behavior

fetch gets matched intercepted

Logs & Screenshots

TypeError: fetch failed
  cause: MockNotMatchedError: Mock dispatch not matched for headers '["accept","application/json","accept-language","*","sec-fetch-mode","cors","user-agent","undici","accept-encoding","br, gzip, deflate"]': subsequent request to origin https://example.com was not allowed (net.connect disabled)

Environment

Node 18.0.0

Additional context

Matching headers with a function works as expected.

[panva]

cc @RafaelGSS

[mcollina]

@KhafraDev could you take a look?

[KhafraDev]

I'll look into it 👍

I don't think this is caused by one of my changes though, I never really used/touched the matching section.

edit: I found the bug, it was caused by the recent changes to the Headers structure in fetch. I should have a PR relatively soon.