`readable.unshift` can take a string as an argument, but that
string wasn't being converted to a Buffer, which caused a
<TypeError: Argument must be a buffer> in some cases. Also if a
string was passed, that string was coerced to utf8 encoding.
A second optional argument `encoding` was added to `unshift` to
fix the encoding issue.
Fixes: https://github.com/nodejs/node/issues/27192
PR-URL: https://github.com/nodejs/node/pull/27194
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>