Improve transliteration support in Strings::toAscii

There are some cases where the ICONV and ICU transliteration will be different. One of them is in the webalize test - ICONV: `"¼"` -> `"1/4 "`, ICU: `"¼"` -> `"1/4"`.

Transliterator is available since PHP 5.4 with intl extension enabled