docs($cacheFactory): prevent example breaking on key update

The example for $cacheFactory breaks when a user tries to update a value for a key.
Setting a new value for an existing key results in duplicate key entries in the key array, thus
breaking the ng-repeat directive. With this fix the key is only added if it isn't contained in the
cache.

Closes #8214

src/ng/cacheFactory.js

@@ -67,8 +67,10 @@
            $scope.keys = [];
            $scope.cache = $cacheFactory('cacheId');
            $scope.put = function(key, value) {
-             $scope.cache.put(key, value);
-             $scope.keys.push(key);
+             if ($scope.cache.get(key) === undefined) {
+               $scope.keys.push(key);
+             }
+             $scope.cache.put(key, value === undefined ? null : value);
            };
          }]);
      </file>