http2: set decodeStrings to false, test

Set writableStream decodeStrings to false to let the
native layer handle converting strings to buffer.

PR-URL: https://github.com/nodejs/node/pull/15140


Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
Reviewed-By: Claudio Rodriguez <cjrodr@yahoo.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>