test: http outgoing _headers setter null

Co-authored-by: Qingyu Deng <i@ayase-lab.com>

PR-URL: https://github.com/nodejs/node/pull/38881


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Zijian Liu <lxxyxzj@gmail.com>
Reviewed-By: Darshan Sen <raisinten@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>