Replace noSender in DeadLetterActorRef, see #2882
(cherry picked from commit 223865d67297801de48b728ec57c5b965c9b9ca9)
Merge pull request #1026 from akka/wip-2882-deadletter-null-sender-2.1-patriknw

Replace noSender in DeadLetterActorRef, see #2882