Update punycode.php

This change fixes the issue where an url processed by the JStringPunycode class gets it's fragment element stripped. Since this code is widely used, any url submitted through a form in which the field type is set to url, validated as an url and filtered as an url will have it's fragment part stripped thus making it impossible to link to a specific anchor on a page. This is a pull request that fixes the issue: http://issues.joomla.org/tracker/joomla-cms/4357