IterateeSubscriber feeds EOF into the iteratee onComplete

[wsargent]

This shouldn't be done, for example, it breaks the following:

Streams.publisherToEnumerator(somePublisher) >>> Enumerator("foo")

If the IterateeSubscriber (created by the PublisherEnumerator) feeds EOF when the publisher is done, then that means the concatenated enumerator can never be reached because the iteratee it feeds into will go done when it receives the EOF.

Moved from https://github.com/playframework/playframework/issues/4956