Never onCompleting Publishers are now suppported

This is based on the https://github.com/reactive-streams/reactive-streams/pull/174 PR (depends on it), so please only review commit: https://github.com/ktoso/reactive-streams/commit/3f68f716e5cd7d8229c1910b0f804e7a0eac4538 as part of *this* pull request.

This adds support and docs for publishers which are not able to onComplete.

We can test such publishers by either slapping a `take(n)` after the publisher (but then you're testing the take), or directly (this proposal). There's quite a few tests which do not need completion signals - I have marked them explicitly now, such that implementers can opt out of them (see README).