3.0 Avoid multiple RunTimeException

In addition to refactor header sending #4597, as tested with @lorenzo , we need also to stop the Exception from being thrown by session.
here was the previous result when trying to do `debug(`ss') from a controller :
 
![capture d ecran 2014-09-13 15 44 02](https://cloud.githubusercontent.com/assets/4977112/4260864/3cae4214-3b52-11e4-9a1f-efabbb6cb000.png)

Any reason why just `$this->_started` is checked instead of using `started()` which checks for `ession_status() === \PHP_SESSION_ACTIVE` too?