util: improve readability of normalizeEncoding

* Improve readability of util.normalizeEncoding
  and add some comments
* Add a benchmark for util.normalizeEncoding

PR-URL: https://github.com/nodejs/node/pull/10439


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Andreas Madsen <amwebdk@gmail.com>