Google Firebase Database, serialize and unserialize a ServerValue.TIMESTAMP with the same Model

gistfile1.txt

```java
// Model
public class User {
    public Object timestamp; // Server time.
    ...
}

// Example code
public void test() {

    // Save
    user.timestamp = ServerValue.TIMESTAMP;
    FirebaseDatabase...setValue(user);
    
    // Load
    FirebaseDatabase...addListenerForSingleValueEvent(DataSnapshot dataSnapshot) {
        User user = dataSnapshot.getValue(User.class);
        ...
        if (user.timestamp instanceof Long) {
            Timber.w("read_value = %s", (Long) user.timestamp);
        }
        else {
            Timber.e("Unexpected timestamp type for %s", user.timestamp);
        }
    }
    
}
```