Introduce serendipity_specialchars-wrapper for encoding bug (#236)
PHP 5.4 sets UTF-8 as the default for htmlspecialchars, htmlentities and html_entity_decode. The first two will echo an empty string when given a string with umlauts. This commits introduces serendipity_specialchar-wrapper that are meant to be a temporary solution for the s9y-core until PHP 5.6 fixed the bug, so the native charset option of s9y continues to work.