Fixes issues caused by bug #835 fix, no longer able to terminate silently

Fixes issues caused by https://github.com/yiisoft/yii/issues/835 where it was no longer possible to terminate the application silently.  E.g. when sending non-HTML (JSON, files, images, etc) and debug info is on, many people use exit to avoid output.

See http://www.yiiframework.com/forum/index.php/topic/47227-issues-with-fixed-bug-835-onendrequest/