http: allow passing array of key/val into writeHead

Enables an optimization when the user already has the headers
in an array form, e.g. when proxying.

PR-URL: https://github.com/nodejs/node/pull/35274


Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>