net: use decodeStrings public API for writable stream

Instead of using an undocumented underscore-prefixed property to
configure the writable stream instance to not encode strings as buffers,
use the public API which is an options property passed to the
constructor.

Refs: https://github.com/nodejs/node/issues/445

PR-URL: https://github.com/nodejs/node/pull/25201


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Anto Aravinth <anto.aravinth.cse@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>