Bug Description

This is the same bug as #1556 but for .reply called with a single callback instead of a callback for data. I believe I have identified the cause:

This callback is called with the raw options passed as opposed to the mutated options that later gets passed to a "data" callback

Reproducible By

import { MockAgent, setGlobalDispatcher } from 'undici'

const mockAgent = new MockAgent()
const mockPool = mockAgent.get('http://localhost')

setGlobalDispatcher(mockAgent)

mockPool.intercept({
  path: '/foo',
  method: 'GET',
}).reply((options) => {
  console.log('headers in reply:', Object.prototype.toString.call(options.headers), options.headers)
  return { statusCode: 200 };
})

await fetch('http://localhost/foo')

Expected Behavior

As specified in the type definitions, I would expect a Headers object when using fetch()

Logs & Screenshots

headers in reply: [object Array] [
  'accept',
  '*/*',
  'accept-language',
  '*',
  'sec-fetch-mode',
  'cors',
  'user-agent',
  'undici',
  'accept-encoding',
  'gzip, deflate'
]

Environment

• Windows 10
• Node v18.15.0
• Undici 5.21.2
• Fetch used is global fetch in node, not imported from undici

Additional context