fix 'Undefined index' exception

If config behavior like that:
```php
'corsFilter' => [
                'class' => Cors::className(),
                'cors' => [
                    'Origin' => ['http://mysite.com'],
                    'Access-Control-Request-Headers' => ['X-Wsse'],
                ],
            ],
```
Got a exception with message 'Undefined index: Access-Control-Allow-Credentials'  because the default settings will be overwritten by user's settings.