Creating a valueOf() method results in a compilation error

Similarly to #367, if you create a method called valueOf(), the Royale compiler spits out an error.
Here's an example Main.as:
```
package {
public class Main {

public function Main () {
console.log("Hi Royale!");
}

public function valueOf():String {
return "Hello!";
}
}
}
```

If you compile with `asjsc -debug=true --targets=js src/Main.as`, the compiler reports this error:
```
src/Main.as(8): col: 19 Error: Overriding a function that is not marked for override

public function valueOf():String {
```

I can submit a PR to change this if you agree it should be changed. Thanks!