fix: resolve #254 - preserve headers set via writeHead

[abhii734]

Bug (compression#254)

When user calls res.writeHead(200, undefined, {'foo': 'bar'}) with custom headers, the compression middleware overwrites those headers when it calls this.writeHead(this.statusCode) without passing the headers.

Fix

Check res._header to detect if writeHead was already called with headers, and skip calling writeHead again if headers exist.

Verification

1. Setup compression middleware with filter returning false (no compression)
2. Call res.writeHead(200, undefined, {'foo': 'bar'})
3. Before fix: headers not set
4. After fix: headers preserved

[UlisesGascon]

Thanks for looking into this @abhii734 and for helping the project!

The headers are dropped inside on-headers before compression ever sees them, so this change cannot restore them, and res.headersSent is already !!res._header on all supported Node versions, so the new condition is equivalent to the old one. The fix is jshttp/on-headers#49.

Closing in favor of that, and #254 stays open until it is released.