title: UIViewController setView nil automatically removes from superview
body: <p>Calling setView on a UIViewController removes the current view from its superview automatically.  I couldn't find this documented. In my case I want to dynamically swap a UIViewController object for another while maintaining my view structure. I was planning to just relink the view to the new controller but alas, that doesn't work.  </p>
  
  <p>In general, automatically removing the view from its superview seems like a sensible decision.  The documentation should reflect this though.</p>
  
  <p>(For anyone thinking it's a really bad idea to swap a view controller object in this way, let me add that the controller I'm swapping in is a subclass of the existing controller.  And this method is great for adding functionality to a view that is an extension of another.)</p>