Fixed ArrayBroadcastChannel receive of stale elements on openSubscription

Author: elizarov

core/kotlinx-coroutines-core/src/main/kotlin/kotlinx/coroutines/experimental/channels/ArrayBroadcastChannel.kt

@@ -72,7 +72,7 @@ class ArrayBroadcastChannel<E>(
 
     override fun openSubscription(): SubscriptionReceiveChannel<E> {
         bufferLock.withLock {
-            val sub = Subscriber(this, head)
+            val sub = Subscriber(this, tail)
             subs.add(sub)
             return sub
         }

core/kotlinx-coroutines-core/src/test/kotlin/kotlinx/coroutines/experimental/channels/BroadcastChannelSubStressTest.kt

@@ -61,6 +61,7 @@ class BroadcastChannelSubStressTest(
                     broadcast.openSubscription().use { sub ->
                         val i = sub.receive()
                         check(i >= last) { "Last was $last, got $i" }
+                        if (!kind.isConflated) check(i != last) { "Last was $last, got it again" }
                         receivedTotal.incrementAndGet()
                         last = i
                     }