Preserve value types in modSystemEvent::output()

Trying to deal with issue described http://forums.modx.com/thread/32541/modx-revo-event-output-issue-in-plugin#dis-post-437548 so now modEvent::output() can be used for setting the output, instead of "semi-private" modEvent::_output, which should generally be used only for internal interactions.

(cherry picked from commit 2e427a5de813d2381bdc5893033e7db960bdbbef)