Widget#data setter misbehaving on iOS 12.4

### Problem description

The `Widget#data` setter stops working after setting values twice. Could reproduce with iOS 12.4. No errors or warnings were printed to the console. Could not reproduce with latest iOS. Could not reproduce on Android.

### Expected behavior

The `Widget#data` setter should always set the given value.

### Environment

- Tabris.js version: 3.8.0
- OS: iOS 12.4

### Code snippet

```js
import {Composite} from 'tabris';

const composite = new Composite();
composite.data.foo = 'foo';
composite.data.foo = 'bar';
composite.data.foo = 'baz';
composite.data.foo = 'baf';
console.log(composite.data.foo); // 'bar'
```