Should be able to reset gradient

If a gradient and a color are set on a rectangle, the gradient is used. This means that if you wish to override the gradient on a component with a color, you have to unset the gradient as well as set the color.

This works if you do 'gradient: null'. However I was under the impression that one should use 'property: undefined' to reset (resettable) properties to their unset state, which is really what one wants here. Then it should be possible to use 'gradient: undefined' as well to reset gradient so that Rectangle uses color.