response组件的format配置无效？ [ 2.0 版本 ]

在进行RESTful时，默认输出格式是xml，我在web.php配置

'response' => ['format' => 'json',],
'urlManager' => [
            'enablePrettyUrl' => true,
            'enableStrictParsing' => true,
            'showScriptName' => false,
            'rules' => [
                ['class' => 'yii\rest\UrlRule', 'controller' => ['player', 'custom']],
            ],
        ],

//    yii\web\Application类

public function handleRequest($request)
    {
        if (empty($this->catchAll)) {
            list ($route, $params) = $request->resolve();
        } else {
            $route = $this->catchAll[0];
            $params = array_splice($this->catchAll, 1);
        }
        try {
            Yii::trace("Route requested: '$route'", __METHOD__);
            $this->requestedRoute = $route;
            $result = $this->runAction($route, $params);
            if ($result instanceof Response) {
                return $result;
            } else {
                $response = $this->getResponse();
                //在这个地方打印$response，它的format属性还是默认值xml，没有改变？
                if ($result !== null) {
                    $response->data = $result;
                }

                return $response;
            }
        } catch (InvalidRouteException $e) {
            throw new NotFoundHttpException($e->getMessage(), $e->getCode(), $e);
        }
    }

为什么response对象的format属性还是xml