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