Serializing `None` doesn't remove the key/value from the registry

[gentoo90]

let v1 = Rectangle {
    coords: Some(Coords { x: 55, y: 77 }),
    size: Size { w: 500, h: 300 },
};
key.encode(&v1).unwrap();

let v2 = Rectangle {
    coords: None,
    size: Size { w: 500, h: 300 },
};
key.encode(&v2).unwrap();

let v3: Rectangle = key.decode().unwrap();
assert_eq!(v3, v2); 

assertion `left == right` failed
  left: Rectangle { coords: Some(Coords { x: 55, y: 77 }), size: Size { w: 500, h: 300 } }
 right: Rectangle { coords: None, size: Size { w: 500, h: 300 } }