fix: preserve writeHead statusText and custom headers (fixes #254)

[Vansh1811]

Problem

When a user calls res.writeHead(200, undefined, {'foo': 'bar'}), the compression middleware was silently losing the custom headers. This happens because:

1. on-headers drops headers when statusText is undefined — it checks typeof arguments[1] === 'string' to determine the header index, treating undefined as "no statusText", but then looks at arguments[1] (which is undefined) instead of arguments[2] for headers
2. The compression middleware calls this.writeHead(this.statusCode) in res.write and res.end without replaying the original statusText/headers, overwriting whatever the user set

Solution

Intercept res.writeHead in the compression middleware to capture user-supplied arguments. In res.write and res.end, if the user previously called writeHead explicitly, replay those captured arguments instead of calling this.writeHead(this.statusCode) alone.

This approach:

• Does not rely on the private res._header property (unlike other proposed fixes)
• Correctly handles all writeHead signatures: (code), (code, message), (code, message, headers), and (code, undefined, headers)
• Preserves backward compatibility

Testing

To reproduce the bug before this fix:

const express = require('express')
const compression = require('compression')
const app = express()
app.use(compression({ filter: () => false }))
app.get('/', (req, res) => {
  res.writeHead(200, undefined, { 'x-custom': 'value' })
  res.end('hello')
})

Before: x-custom header was missing. After this fix, it is correctly sent.

Fixes #254