`prefersStatusBarHidden` in UIViewController don't work.

When set `prefersStatusBarHidden` in `DEMOSecondViewController`, The `self.contentViewController.prefersStatusBarHidden` doesn't work.

Because:
- `self.contentViewController` is a `UINavigationViewController`, have to get visibleViewController vaule.
- when tap left menu second cell, `self.contentViewController` still is firstViewController's navigation.

This pull request already fixed them.