Fixed crash with TableHelper when an event subscriber definition is an array of arrays

Event subscriber definitions (`getSubscribedEvents()`) may be in the following format:
* `array('eventName' => 'methodName')`
* `array('eventName' => array('methodName', $priority))`
* `array('eventName' => array(array('methodName1', $priority), array('methodName2'))`

Using ListenersDebugCommand and having at least one subscriber with the last form (array of arrays), an error is raised with mbstring:

```
Warning: mb_detect_encoding() expects parameter 1 to be string, array given in /Users/lolautruche/workspace/ezsystems/ezpublish/vendor/symfony/sy
  mfony/src/Symfony/Component/Console/Helper/Helper.php line 56
```

This patch fixes that :octocat: 

Only drawback is that it doesn't show the right priority... But this would deserve some refactoring :wink: