Refactor: Removed PreventEnumerationOnEvaluation flag

The fact that internally all unprocessed pipeline output is represented
as an object[] instead of Collection<object> saves a lot of effort and ugly
workarounds.
Therefore the newly introduced and very ugly "PreventEnumerationOnEvaluation" flag
could savely be removed again.
Code is much cleaner now and better to understand.
Added test for issue #116