stream: convert existing buffer when calling .setEncoding

Convert already-stored chunks when `.setEncoding()` is called
so that subsequent `data` events will receive decoded strings,
as they expect.

Fixes: https://github.com/nodejs/node/issues/27932

PR-URL: https://github.com/nodejs/node/pull/27936


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