Can not generate default value for using enum in class

```typescript
enum Fruit {
  Apple = 'apple',
  Orange = 'orange'
}
class Test {
  fruit: Fruit = Fruit.Apple;
}
```
Then I get:
```typescript
{
  fruit: {
    enum: ['apple', 'orange'],
    type: 'string',
  }
}
```
I expect the default can be set in here.