UTF8: fixed typos and bugs

Bugs: 
UTF8::clean():
check UTF8::$server_utf8 not specified
Error result: http://kohanaframework.org/3.3/guide-api/Kohana ErrorException [Fatal Error]: Call to undefined function mb_convert_encoding().

UTF8::$server_utf8:
Set Kohana_UTF8::$server_utf8 not true, because UTF8 use property as UTF8::$server_utf8