fix(backButton): add .header-item class

Add the .header-item class to each of the root child items of a nav
header bar. This gives CSS more power to state what should and should
not be shown during the different states of a transition, specifically
for iOS nav bar transitions.

js/angular/directive/navBackButton.js

@@ -71,7 +71,7 @@ IonicModule
         buttonEle.setAttribute('ng-click', '$ionicGoBack($event)');
       }
 
-      buttonEle.className = 'button back-button hide buttons ' + (tElement.attr('class') || '');
+      buttonEle.className = 'button back-button back-disabled buttons ' + (tElement.attr('class') || '');
       buttonEle.innerHTML = tElement.html() || '';
 
       var childNode;