=act #13966 Change wrong default discardOld in UntypedActor become

(cherry picked from commit 2aada3b5a857552f9d7854a19cd72d77e6a52b9b)
Merge pull request #15096 from akka/wip-13966-become-discardOld-master-patriknw

=act #13966 Change wrong default discardOld in UntypedActor become (for validation)