title: Change the font on the back button of a navigation controller under swift 4
body: <p>Swift 4, iOS 11.2.5 Xcode 9.2</p>
  
  <p>Trying to change the font of the back button. Tried previous solutions found, but none seem to work under Swift 4, iOS 11.2.5 with my configuration, navigation controller within a tab bar controller.</p>
  
  <p>Got this code, the first and last lines work, but the center three do not.</p>
  
  <pre><code>self.navigationController?.navigationBar.titleTextAttributes = [ NSAttributedStringKey.font: UIFont(name: "AvenirNextCondensed-DemiBoldItalic", size: 20)!]
  navigationItem.backBarButtonItem?.setTitleTextAttributes([NSAttributedStringKey.font: UIFont(name: "AvenirNextCondensed-DemiBoldItalic", size: 20)!], for: .normal)
  navigationItem.backBarButtonItem?.setTitleTextAttributes([NSAttributedStringKey.font: UIFont(name: "AvenirNextCondensed-DemiBoldItalic", size: 20)!], for: .highlighted)
  navigationItem.backBarButtonItem?.setTitleTextAttributes([NSAttributedStringKey.font: UIFont(name: "AvenirNextCondensed-DemiBoldItalic", size: 20)!], for: .focused)
  navigationItem.title = "Filter \(titleSelection!) [shake to clear]"
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/yqGJS.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/yqGJS.png" alt="enter image description here"></a></p>
  
  <p>This is in <code>viewDidLoad</code> method. Should this work?</p>