Update Normalizer.php

Normalizer works incorrectly with non-ASCII domains — this is what is expected:
http://www.Яндекс.РФ → http://www.яндекс.рф
http://dev.ŽiŪrKėNaS.lt → http://dev.žiūrkėnas.lt

Additionally, a requirement for "mb_strtolower" function to be used could be added with a line in "composer.json"
````
"ext-mbstring": "*",
````