Void Union Prop Types marked as Required

When setting up a prop type as:

`foo: Foo | void`

I get the following warning in my console:

> Warning: Failed prop type: The prop `foo` is marked as required in `MyComponent`, but it's value is `undefined`.

I can make the warning go away by changing the prop definition to:

`foo? Foo`

but of course this changes the meaning of my type def.