title: UIStackView's own intrinsicContentSize
body: <p>I'm using the UIStackView with the following configuration:</p>
  
  <pre><code>let contentView = UIStackView()
  contentView.distribution = .EqualSpacing
  contentView.alignment = .Center
  contentView.spacing = horizontalSpacing
  </code></pre>
  
  <p>Each of the elements has it's own <code>intrinsicContentSize</code> so it should be possible for the <code>UIStackView</code> to provide its own <code>intrinsicContentSize</code>. The documentation states that the <code>spacing</code> is used as a minimum spacing. </p>
  
  <p>Example:</p>
  
  <pre><code>view1: width=10
  view2: width=15
  spacing = 5
  [view1(10)]-5-[view2(15)]
  </code></pre>
  
  <p>The <code>intrinsicContentSize.width</code> of the stackView should be <code>30</code>.</p>
  
  <p>Instead I get:</p>
  
  <pre><code>▿ CGSize
   - width : -1.0
   - height : -1.0 { ... }
  </code></pre>
  
  <p>which tells me that the  <code>intrinsicContentSize</code> cannot be provided.</p>
  
  <p>Does anyone of you know if I doing something wrong, if the behaviour is intended or if this is a bug?</p>