ReplyHeaders type fixed

[mlegenhausen]

Following type fixes does this PR include:

• string[] removed from ReplyHeaderValue and ReplyHeaderFunction. The resulting IncomingMessage.rawHeaders value would else be invalid. See example below.
• ReplyHeaderFunction[] replaced with just string[]. This actually leads to an runtime error when running nock.

See following examples

import http from 'node:http';
import nock from 'nock';

nock('http://github.com')
  .get('/nock/nock')
  .reply(200, 'Hello from GitHub!', {
    'X-My-Headers': ['My Header value 1', 'My Header value 2'],
  });

const req = http.request('http://github.com/nock/nock', (res) => {
  console.log(res.rawHeaders);
});

req.end();

Output:

[ 'X-My-Headers', [ 'My Header value 1', 'My Header value 2' ] ]

Which is an invalid rawHeaders value. The node docs define a string[]: https://nodejs.org/docs/latest/api/http.html#messagerawheaders

import http from 'node:http';
import nock from 'nock';

nock('http://github.com')
  .get('/nock/nock')
  .reply(200, 'Hello from GitHub!', [() => 'Test1', () => 'Test2']);

const req = http.request('http://github.com/nock/nock', (res) => {
  console.log(res.rawHeaders);
});

req.end();

This results in an exception:

TypeError: name.toLowerCase is not a function
    at addHeaderLine (/node_modules/nock/lib/common.js:339:20)
    at /node_modules/nock/lib/common.js:280:5
    at forEachHeader (/node_modules/nock/lib/common.js:396:5)
    at Object.headersArrayToObject (/node_modules/nock/lib/common.js:279:3)
    at Interceptor.reply (/node_modules/nock/lib/interceptor.js:154:27)
    at file:///playground/dist/main.js:5:6
    at ModuleJob.run (node:internal/modules/esm/module_job:218:25)
    at async ModuleLoader.import (node:internal/modules/esm/loader:329:24)
    at async loadESM (node:internal/process/esm_loader:28:7)
    at async handleMainPromise (node:internal/modules/run_main:113:12)