DropDownMenu should not auto selectedIndex

[cuongcua90]

I think when we define a DropDownMenu without selectedIndex we can show text for example 'Choose' .
It not necessary to default the index is 0, and it's difficult if we use onChange event if the index we want change is first.

[hai-cea]

@cuongcua90 I think this is fixed with the new SelectField component. See #846