fix: handle undefined statusText in writeHead arguments

[mahmoodhamdi]

Summary

Fixes argument parsing in setWriteHeadHeaders to correctly handle writeHead(statusCode, undefined, headers) — where the statusText is explicitly passed as undefined or null.

Problem

When res.writeHead(200, undefined, { 'X-Custom': 'value' }) is called through on-headers, the headers object at the third argument position is silently dropped.

Root cause: The argument parsing logic determines the header index based only on whether arguments[1] is a string:

var headerIndex = length > 1 && typeof arguments[1] === 'string'
    ? 2
    : 1

When arguments[1] is undefined, it's not a string, so headerIndex = 1. Then arguments[1] (which is undefined) is read as the headers, and the actual headers at arguments[2] are never accessed.

Without on-headers: Native Node.js writeHead(200, undefined, headers) works correctly.
With on-headers: Headers are silently dropped.

This was reported as expressjs/compression#254, where the compression middleware (which uses on-headers) caused headers to be lost when the Vercel AI SDK called writeHead(status, undefined, headers).

Fix

When there are more than 2 arguments, the headers are always at index 2 regardless of whether the second argument is a string, matching Node.js native writeHead behavior:

var headerIndex = length > 1 && typeof arguments[1] === 'string'
    ? 2
    : length > 2
      ? 2
      : 1

Testing

Added 2 new tests:

• writeHead(status, undefined, obj) — headers are set correctly
• writeHead(status, null, obj) — headers are set correctly

All 26 tests pass. Lint clean.