title: Set UIButton width 0 when no title with AutoLayout
body: <p>I need set UIButton title dynamically and its width fit it's content ,
  But when I set title with @"" or nil string , it's width still not be zero , it seems about 30 pt.</p>
  
  <p>I had tried to set its Intrinsic Size Placeholder to 0 at Storyboard and sizeToFit method , But both didn't work.</p>