More fine-tuning of the specs, fixes of mistakes uncovered by akka-streams

+ checking 3.9 in PublisherVerification
+ better error messages on expectThrowingOf
+ fixed 2.5 tests – they were checking the wrong things
+ checking 1.13 must be optional, because some publishers dont support fan-out
+ inverse condition in 3.13 to avoid NPE, and report a nice error
+ checking 1.7 should give more demand, then pull based publishers will
  notice that their source has been drained and complete the downstream
+ checking 1.13 should give more demand, as if the publisher is
  pull-based, it would not notice (and has 0 buffer space), it would not
  notice that it's source has been drained, and would not complete the
  stream.