Add opportunity to directly assign `undefined` for optional properties

Right now, we cannot assign the undefined property value, even if it is optional. So it forces to write additional boilerplate to return empty object instead of undefined property. For example:

``` js
const getProperty = () => {
if (condition) {
return { property };
}

return {};
}

...

request: {
...getProperty()
...
}

// instead of

request: {
property: condition ? property : undefined
}

```