Refactor toggleHeading to be clearer about it's intentions

isOpen was initially set, to the *current* state. Subsequent controls
were inverting that. If we invert it at the beginning this becomes clearer
Note - also renamed to setOpen to signify intent.

lib/assets/javascripts/_modules/collapsible-navigation.js

@@ -47,14 +47,15 @@
     }
 
     function toggleHeading ($topLevelItem) {
-      var isOpen = $topLevelItem.hasClass('is-open')
+      var setOpen = !$topLevelItem.hasClass('is-open')
+
       var $heading = $topLevelItem.find('> a')
       var $button = $topLevelItem.find('.collapsible__toggle')
       var $toggleLabel = $topLevelItem.find('.collapsible__toggle-label')
 
-      $topLevelItem.toggleClass('is-open', !isOpen)
-      $button.attr('aria-expanded', isOpen ? 'false' : 'true')
-      $toggleLabel.text(isOpen ? 'Expand ' + $heading.text() : 'Collapse ' + $heading.text())
+      $topLevelItem.toggleClass('is-open', setOpen)
+      $button.attr('aria-expanded', setOpen ? 'true' : 'false')
+      $toggleLabel.text(setOpen ? 'Collapse ' + $heading.text() : 'Expand ' + $heading.text())
     }
 
     function openActiveHeading () {