REST: Receive UTF-8 Strings

This pull request implements [my suggestion](https://github.com/xp-framework/xp-framework/issues/324#issuecomment-22380951) to receiving utf-8 in #324 enables the following:

```php
#[@webmethod...]
public function getDomain($domain) {
  // $domain = string, xp::ENCODING = "iso-8859-1" bytes
}

#[@webmethod...]
public function getDomain(String $domain) {
  // $domain = lang.types.String, utf-8 bytes
  // Can be converted to iso-8859-1 using getBytes() / string casting
}
```