Add native_utf8, fix _load, clean, is_ascii

`UTF8::native_utf8()`: added to preserve purity of class file

`UTF8::_load()`: need use require_once because `UTF8::$called` is public property:
~~~
unset(UTF8::$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