Remove type-attr from <select> b/c it's invalid

fixes #1776

inc/Form/DropdownElement.php

@@ -21,6 +21,7 @@ class DropdownElement extends InputElement {
      */
     public function __construct($name, $options, $label = '') {
         parent::__construct('dropdown', $name, $label);
+        $this->rmattr('type');
         $this->options($options);
     }