=con #3975 Check for wrong id in cluster sharding

* Also, a watch leftover from ticket 3882

(cherry picked from commit cbc9dc535c0692a7df00bfb7292e62de1bed7e3f)

Conflicts:
	akka-contrib/src/main/scala/akka/contrib/pattern/DistributedPubSubMediator.scala
Merge pull request #2122 from akka/wip-3975-sharding-invalid-id-master-patriknw

=con #3975 Check for wrong id in cluster sharding (forward port)