Fix to avoid a cyclic reference in UIBarButtonItem.

Push this controller on a navigation controller stack:

```ruby
class LeakyController < UIViewController
  def viewWillAppear(animated)
    super
    self.navigationItem.leftBarButtonItem = UIBarButtonItem.done { go_back }
  end

  def go_back
    navigationController.popViewControllerAnimated(true)
  end

  def dealloc
    puts "LeakyController.dealloc"
  end
end
```

When `popViewControllerAnimated` is called, `LeakyController` will not be released if `UIBarButtonItem` has a strong ref on the action block.