fix(ngAnimate): guard $document[0].hidden access in case it was mocked out

Some tests mock out  and now that we always access the hidden property, existing
tests can get broken. This change keeps the existing tests working.

src/ngAnimate/animateQueue.js

@@ -331,7 +331,7 @@ var $$AnimateQueueProvider = ['$animateProvider', function($animateProvider) {
 
       var isStructural = ['enter', 'move', 'leave'].indexOf(event) >= 0;
 
-      var documentHidden = $document[0].hidden;
+      var documentHidden = animationsEnabled && $document[0].hidden;
 
       // this is a hard disable of all animations for the application or on
       // the element itself, therefore  there is no need to continue further