Use Default Value for Undefined Props

Currently, default props as defined by `getDefaultProps` are only used if a prop is not specified (using `X in Y` check).

However, this makes it difficult for composing components to pass along the state of not being defined, for example:

  render: function() {
    // If `this.props.someProp` is not set, `InnerThing` should use the default value.
    return <InnerThing someProp={this.props.someProp} />;
  }

This changes the requirement for falling back to the default value to an undefined check (using `typeof X === 'undefined'`).