Use getInternalRequest to return Symfony\Component\BrowserKit\Request

Attempting to use the REST module with a Symfony 2.3 client results in the following error:

> Error: Call to undefined method Symfony\Component\HttpFoundation\Request::getCookies() in codeception/codeception/src/Codeception/Module/REST.php line 352

As in #621,  Symfony 2.3 ```Symfony\Component\BrowserKit\Client::getRequest()``` returns ```Symfony\Component\HttpFoundation\Request```, which does not have a ```getCookies``` method. Use ```getInternalRequest``` to return ```Symfony\Component\BrowserKit\Request```, which does.