Use UTF-8 encoding for htmlspecialchars. See #36.

Prior to PHP 5.4.0 the default encoding for `htmlentities()`
and `htmlspecialchars` is "ISO-8859-1". For PHP 5.4+ is "UTF-8".

This ensures always the right encoding is used no matter the PHP version
and the locale settings.
Merge pull request #37 from hkdobrev/htmlspecialshars-utf8