title: Stack View constrained to margins leaves no margin
body: <p>I got some problem that I'm not sure how to solve...
  I'm working on some lessons which are a little bit old (they are done in Swift 3), and I got a problem with constraints and margins. I'm just following the lesson and it says that for that stack view I need to put following constraints: 
  <a href="https://i.stack.imgur.com/UTcw5.png" rel="noreferrer"><img src="https://i.stack.imgur.com/UTcw5.png" alt="Given constraints"></a></p>
  
  <p>After adding constraints to stack view I got this result:</p>
  
  <p><a href="https://i.stack.imgur.com/nT2tH.png" rel="noreferrer"><img src="https://i.stack.imgur.com/nT2tH.png" alt="Result"></a></p>
  
  <p>and this is the result I expected:</p>
  
  <p><a href="https://i.stack.imgur.com/leI0i.png" rel="noreferrer"><img src="https://i.stack.imgur.com/leI0i.png" alt="Expected Result"></a></p>
  
  <p>So my question is Why that stack view goes over margins, and how to fix it.
  (reminder lesson was in Swift 3 and they got stack view inside the margins)</p>