fix: preserve writeHead headers when statusMessage is undefined

[dyk1454683243-sudo]

Fixes #254

Problem

res.writeHead(statusCode, undefined, headers) (and the same call with null as the status text) silently drops the headers when compression is mounted. Native http.ServerResponse applies those headers and uses the default reason phrase for the status code (Created for 201, OK for 200). It does not stringify undefined into the status line.

This is the Vercel AI SDK path: response.writeHead(status ?? 200, statusText, headers) when statusText is omitted.

Root cause: on-headers@1.1.x picks the headers argument with typeof arguments[1] === 'string'. A non-string second argument is treated as the headers object, so arguments[2] is never read. jshttp/on-headers#49 is still open; #254 stays broken until that ships or compression compensates.

Fix

Wrap res.writeHead outside on-headers and rewrite the 3-argument form the same way Node does (obj ??= reason):

• writeHead(code, undefined|null, headers) → writeHead(code, headers)
• writeHead(code, headers, undefined|null) → writeHead(code, headers) (keeps the fix: preserve headers when statusText is undefined in writeHead #282 regression from coming back)

String status messages, 1- and 2-argument calls, and the empty-string reason phrase are unchanged.

Tests

• Headers + gzip for undefined / null status text
• Headers when the 3rd argument is undefined / null (the overload fix: preserve headers when statusText is undefined in writeHead #282 dropped)
• String status message, filter: false, and write() after writeHead
• Wire statusMessage matches native Node (Created / OK / custom; never "undefined")

Notes

Earlier compression PRs for this issue (#273, #274, #282) were closed in favor of on-headers#49. This change is a small, local workaround so #254 can close without waiting on that release, and it does not regress the 3-arg headers-in-the-second-slot overload.