[ JS ] Wrong value for byte (0xFF) when increment

The following example will return 0 for eval, hashlink, java , but 256 for javascript
```haxe
var key = haxe.io.Bytes.ofHex('b5cb4d8dd2c2fc705e10a825000043ff');
key.set(15, key.get(15) + 1);
trace(key.get(15));
```
This will take 0xFF ( last byte from ```key``` and will add +1 . Result should be 0