Type is mismatch for IsCheckedProperty of ToggleButton

[nishy2000]

Type of ToggleButton.IsCheckedProperty is 'bool?'.
But the type of value is treated as "bool" type in ToggleButtonHelper.CoerceIsChecked() and ToggleButtonHelper.OnIsCheckedChanged().

These functions is used in ToggleButton.cs

        static ToggleButton()
        {
            Type type = typeof(ToggleButton);
            DefaultStyleKeyProperty.OverrideMetadata(type, new FrameworkPropertyMetadata(type));
            IsCheckedProperty.OverrideMetadata(type, new FrameworkPropertyMetadata(ToggleButtonHelper.OnIsCheckedChanged, ToggleButtonHelper.CoerceIsChecked));
            ContextMenuService.Attach(type);
            ToolTipService.Attach(type);
            StyleProperty.OverrideMetadata(type, new FrameworkPropertyMetadata(null, new CoerceValueCallback(OnCoerceStyle)));
        }