title: UITableView tableViewHeader does not respect the table safearea
body: <p>I set constraints of a vStack (UIStackView) relative to my view <code>safeAreaLayout</code>.
  The view is used as the <code>tableHeaderView</code> of a <code>UITable</code>.
  The parent safeArea is not respected by the headerView, resulting in misalignment.</p>
  
  <p>How does one fix this ?</p>
  
  <p>Here's the constraints set-up, it's done in the <code>init</code> of the view</p>
  
  <pre class="lang-swift prettyprint-override"><code>let vStack = UIStackView(frame: .zero)
  vStack.axis = .vertical
  vStack.alignment = .leading
  vStack.translatesAutoresizingMaskIntoConstraints = false
  addSubview(vStack)
  
  if #available(iOS 11.0, *) {
      vStack.leadingAnchor.constraint(equalTo: safeAreaLayoutGuide.leadingAnchor).isActive = true
      vStack.trailingAnchor.constraint(equalTo: safeAreaLayoutGuide.trailingAnchor).isActive = true
  } else {
      vStack.leadingAnchor.constraint(equalTo: layoutMarginsGuide.leadingAnchor).isActive = true
      vStack.trailingAnchor.constraint(equalTo: layoutMarginsGuide.trailingAnchor).isActive = true
  }
  
  vStack.topAnchor.constraint(equalTo: topAnchor, constant: 48).isActive = true
  vStack.bottomAnchor.constraint(equalTo: bottomAnchor, constant: -40).isActive = true
  </code></pre>