DefineMap set new prop to undefined

The issue #408 makes this:

const obj = new DefineMap({});
obj.on("foo", (ev, newVal, oldVal) => {
    console.log(ev.patches[0]) // => {type: "add", key: "foo", value: undefined}
});
obj.set("foo", undefined); 

Dispatch [{ type: "add", key: "foo", value: undefined }]