Nested properties and prop-types

Is it an oversight or a limitation that for the `react/prop-types` rule, no warning is emitted for missing nested properties.

For example, with:

``` js
propTypes: {
  a: React.PropTypes.shape({
    b: React.PropTypes.shape({
      c: React.PropTypes.func
    })
  })
}
```

I get no warning when I use `this.props.a.b.d`. 

Thank you!
