Fix adding defaultCssClass

Because method class() is not used for css. For adding attributes are used methods add<attributeName>. In this case addClass('class').

Author: mdsystem

src/Selectoo.php

@@ -410,7 +410,7 @@ public function getControlPart()
 			$element->multiple(true);
 		}
 		if ($this->getDefaultCssClass() !== null) {
-			$element->class(($element->class ? $element->class . ' ' : '') . $this->getDefaultCssClass());
+			$element->addClass($this->getDefaultCssClass());
 		}
 		return $element;
 	}