MaterialDatePicker.getValue shows initial value after detached

[gkresic]

Steps to reproduce:

1. Create and attach MaterialDatePicker
2. Set value using setValue to <DATE_1>
3. Change value using picker to <DATE_2>
4. Read value using getValue and <DATE_2> is correctly returned
5. Detach widget from DOM
6. Read value using getValue and (old) <DATE_1> is incorrectly returned
7. At this point, getDate returns correctly <DATE_2>

Is this intentional or a bug?