Allow default value encoded as JSON

I have wirted like this。
```
@JsonKey(name: 'crop_area', defaultValue: {'top': 0.0, 'left': 0.0, 'right': 0.0, 'bottom': 0.0})
Crop_area cropArea;
```
I have defined Crop_area as model，but it dose not work when I use `CustomItem.fromJson({...})`，`{...}`not include `crop_area`