fixed#134 数据绑定失效

src/Shared/HandyControl_Shared/Controls/Input/DateTimePicker.cs

@@ -82,8 +82,8 @@ namespace HandyControl.Controls
             InitCalendarWithClock();
             CommandBindings.Add(new CommandBinding(ControlCommands.Clear, (s, e) =>
             {
-                ClearValue(SelectedDateTimeProperty);
-                ClearValue(TextProperty);
+                SetCurrentValue(SelectedDateTimeProperty, null);
+                SetCurrentValue(TextProperty, "");
                 _textBox.Text = string.Empty;
             }));
         }