Adjust scheduler isCancelled, see #2903

* isCancelled shall return false if the task is expired (run)
  before cancel() is performed
(cherry picked from commit 0c93ca317ef9085c3928e761843aa6d60912741c)

Conflicts:

	akka-actor-tests/src/test/scala/akka/actor/SchedulerSpec.scala
	akka-actor/src/main/scala/akka/actor/Scheduler.scala
Merge pull request #1159 from akka/wip-2903-cancel-scheduled-task-master-patriknw

Adjust scheduler isCancelled, see #2903