FlattenException incompatibility with Sf2.3

Hey guys, not sure if this is the best approach, as it requires dropping backwards compatibility.

Basically in Symfony2.3, afaik, they introduce a new debug component. The FlattenException has been moved there, and for BC what they did is to keep the one in HttpFoundation, by making it extend the one in Debug.

That's cool but then this Exception controller intercepts internal exceptions from Sf2, and the Exceptions thrown are the ones from the Debug component. Therefore you get amazing things like:

```
PHPUnit_Framework_Error: Argument 2 passed to FOS\RestBundle\Controller\ExceptionController::showAction() must be an instance of Symfony\Component\HttpKernel\Exception\FlattenException, instance of Symfony\Component\Debug\Exception\FlattenException given)
```

I just noticed it after updating our libs via composer. Am I missing something?

Dunno your thoughts about this, just suggesting to find a quick fix :)