test: increase coverage of string-decoder

Make use of Arrow Function.
Add normalizeencoding's test.
normalizeEncoding:
https://github.com/nodejs/node/blob/master/lib/string_decoder.js#L9

PR-URL: https://github.com/nodejs/node/pull/10863


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>