Update UTF8 classes

UTF8::native_utf8(): added to preserve purity of class file

UTF8::_load(): need use require_once because `UTF8::$called` is publiс property:
~~~
unset(UTF8::$called['strlen']);
UTF8::strlen($str);
~~~

UTF8::clean():
reordered UTF8::native_utf8() and UTF8::is_ansii() - basic condition "server_utf8 = true".

UTF8::is_ascii(): added support for traversable objects