title: UIToolbar doesn't show UIBarButtonItem with custom view (only below iOS 11)
body: <p>I'm testing a simple <code>UIToolbar</code>, with two <code>UIBarButtonItem</code> as items, one built using <code>initWithCustomView:</code> method, the other one using <code>initWithTitle:style:target:action:</code> method.</p>
  
  <p>The second button cand be viewed on the toolbar, but the first one is missing.</p>
  
  <p><strong>The problem occurs only on iOS 9 and iOS 10. It doesn't occur on iOS 11.</strong></p>
  
  <p>Does anyone have any idea?</p>
  
  <pre><code>UIToolbar *toolbar = [[UIToolbar alloc] init];
  [toolbar setTranslatesAutoresizingMaskIntoConstraints:NO];
  
  UIButton *button = [UIButton buttonWithType:UIButtonTypeCustom];
  [button setTitle:@"one" forState:UIControlStateNormal];
  [button setImage:[UIImage imageNamed:audioOnIcon] forState:UIControlStateNormal];
  
  [toolbar setItems:[NSArray arrayWithObjects:
                     [[UIBarButtonItem alloc] initWithCustomView:button],
                     [[UIBarButtonItem alloc] initWithTitle:@"title" style:UIBarButtonItemStylePlain target:self action:nil],
                     nil]];
  
  [toolbar setBackgroundImage:[UIImage new] forToolbarPosition:UIBarPositionAny barMetrics:UIBarMetricsDefault];
  [toolbar setShadowImage:[UIImage new] forToolbarPosition:UIBarPositionAny];
  toolbar.accessibilityIdentifier = toolbarIdentifier;
  
  [self.view insertSubview:toolbar atIndex:[self getSubviewIndex:toolbar.accessibilityIdentifier]];
  
  [self.view addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"H:|-0-[subview]-0-|" options:NSLayoutFormatDirectionLeadingToTrailing metrics:nil views:@{ @"subview": toolbar}]];
  [self.view addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"V:[subview]-padding-|" options:NSLayoutFormatDirectionLeadingToTrailing metrics:@{ @"padding": @(2 * PADDING) } views:@{ @"subview": toolbar }]];
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/AwGgc.png" rel="noreferrer"><img src="https://i.stack.imgur.com/AwGgc.png" alt="screenshot from iPad 2"></a></p>