Abstract DataObject as property type generate an impossible converter

If you make a DataObject of this kind

```java
@DataObject(generateConverter = true)
public class TestObjectWithInterfaceField {

private TestInterface anInterface = new TestInterfaceImpl();

public TestObjectWithInterfaceField() {
}

public TestObjectWithInterfaceField(JsonObject object){

}

public TestInterface getAnInterface() {
return anInterface;
}

public void setAnInterface(TestInterface anInterface) {
this.anInterface = anInterface;
}
}
```

with TestInterface as :

```java
@DataObject
public interface TestInterface {

String getString();

}
```

the generated converted is :

```java
public class TestObjectWithInterfaceFieldConverter {

public static void fromJson(JsonObject json, TestObjectWithInterfaceField obj) {
if (json.getValue("anInterface") instanceof JsonObject) {
obj.setAnInterface(new TestInterface((JsonObject)json.getValue("anInterface")));
}
}

public static void toJson(TestObjectWithInterfaceField obj, JsonObject json) {
}
}
```
Obviously, converter isn't usable, even valid... is it fixable?