In datetime_widget in 'datetimepicker' mode, display date if it has value

If I configure a field as:
    
            ->add('createdAt', null, array(
                  'required' => 'required',
                  'widget' => "single_text",
                  'format' => "yyyy-MM-dd HH:mm",
                  'datetimepicker' => true
              ));

It is allright in creation mode. But in edit the value of my field createdAt was not displayed. This fix is about to show the initial value.