Bug in CWebUser#renewCookie if config file contains identityCookie params

Having the following configuration:
```php
'user' => array(
  'allowAutoLogin' => true,
  'autoRenewCookie' => true,
  'identityCookie' => array('domain' => '.example.com')
)
```
The expected behavior is that on every request, the identity cookie will be renewed, and the domain of the cookie will be set to ".example.com".

However, this is only true for the first request, which calls CWebUser#createIdentityCookie. Any call to CWebUser#renewCookie does not honor the identityCookie parameter in the configuration file.
Since the cookie sent by the browser back to the server does not contain any domain information (only name = value), this causes the server to start renewing the cookie with the default domain "example.com".

Operating System: Mac OS X 10.7.5
Web Server: nginx 1.2.8 with php-fpm (php 5.4.13)
Browser: Chrome 26
Yii Version: 1.1.13