http: refactor outgoing headers processing

Use a shared function, for..in instead of Object.keys, do less work in
`setHeader` and instead defer some of it until later, and other minor
changes to improve clarity, as well as a slight boost in performance.

PR-URL: https://github.com/nodejs/node/pull/20250


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>