Fixed 404 exception for TbDatePicker.php when app locale is "en"

```
2013/03/05 18:17:15 [error] [exception.CHttpException.404] exception 'CHttpException' with message 'Unable to resolve the request "assets/f44661cf/js/locales/bootstrap-datepicker.en.js".' in /var/www/..../framework/web/CWebApplication.php:286
```