Undici v19 support

repro.js

const Fastify = require('./fastify')
const { Client } = require('undici')
const assert = require('assert')

const fastify = Fastify({
  return503OnClosing: true,
  forceCloseConnections: false
})

fastify.get('/', (req, reply) => {
  fastify.close()
  reply.send({ hello: 'world' })
})

fastify.listen({ port: 0 }, async err => {
  if (err) throw err
  const instance = new Client('http://localhost:' + fastify.server.address().port, {
    pipelining: 1
  })

  const codes = [200, 503]
  for (const code of codes) {
    instance.request(
      { path: '/', method: 'GET' }
    ).then(data => {
      console.log('done', data.statusCode)
      assert.equal(data.statusCode, code)
    }).catch((e) => {
      console.error('fail', e)
    })
  }
  instance.close(() => {
    console.log('done')
  })
})