Better fix for Dropdown container options

Better fix for #3443 that does not break BC and allows widget to be extensible.

Dropdown.php

@@ -40,7 +40,12 @@ class Dropdown extends Widget
      */
     public $encodeLabels = true;
 
-
+    /**
+     * @var array the HTML attributes for the widget container tag.
+     * @see \yii\helpers\Html::renderTagAttributes() for details on how attributes are being rendered.
+     */
+    protected $_containerOptions = [];
+    
     /**
      * Initializes the widget.
      * If you override this method, make sure you call the parent implementation first.
@@ -49,6 +54,7 @@ class Dropdown extends Widget
     {
         parent::init();
         Html::addCssClass($this->options, 'dropdown-menu');
+        $this->_containerOptions = $this->options;
     }
 
     /**
@@ -56,17 +62,16 @@ class Dropdown extends Widget
      */
     public function run()
     {
-        echo $this->renderItems($this->items, $this->options);
+        echo $this->renderItems($this->items);
     }
 
     /**
      * Renders menu items.
      * @param array $items the menu items to be rendered
-     * @param array $containerOptions the HTML attributes for the widget container tag
      * @return string the rendering result.
      * @throws InvalidConfigException if the label option is not specified in one of the items.
      */
-    protected function renderItems($items, $containerOptions)
+    protected function renderItems($items)
     {
         $lines = [];
         foreach ($items as $i => $item) {
@@ -88,13 +93,13 @@ class Dropdown extends Widget
             $linkOptions['tabindex'] = '-1';
             $content = Html::a($label, ArrayHelper::getValue($item, 'url', '#'), $linkOptions);
             if (!empty($item['items'])) {
-                unset($containerOptions['id']);
+                unset($this->_containerOptions['id']);
-                $this->renderItems($item['items'], $containerOptions);
+                $this->renderItems($item['items']);
                 Html::addCssClass($options, 'dropdown-submenu');
             }
             $lines[] = Html::tag('li', $content, $options);
         }
 
-        return Html::tag('ul', implode("\n", $lines), $this->options);
+        return Html::tag('ul', implode("\n", $lines), $this->_containerOptions);
     }
 }