Does not respond Map field ?

Hi there. I find that respond will not show Map field.

Doamin: 

```groovy
class Data {
    Map data

    static mapping = {
        data type: JsonbMapType
    }
}
```

controller:
```groovy
def show(Integer id) {
    respond Data.get(id)
}
```

Will not show `data` field as it's a Map field. What's wrong with me ?