http: improve for-loop readability in _http_outgoing.js

PR-URL: https://github.com/nodejs/node/pull/26408


Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>