Fixed modifying the page output through the on_page_output event.

As discussed with @aembler here:
http://www.concrete5.org/community/forums/5-7-discussion/how-to-modify-page-contents-within-a-package/#688821

I'm not sure whether the own event class would be required here but on the other hand, I don't really see any benefits in that either. Maybe checking the event class' type during the event callback but we can also check `if ($ev->hasArgument('contents'))`.