Make unsubscribe idempotent.

Author: benjchristensen

language-adaptors/rxjava-scala/src/main/scala/rx/lang/scala/Subscription.scala

@@ -64,7 +64,7 @@ object Subscription {
       def isUnsubscribed = unsubscribed.get()
 
       val asJavaSubscription = new rx.Subscription {
-        def unsubscribe() { u; unsubscribed.set(true) }
+        def unsubscribe() { if(unsubscribed.get()) { u ; unsubscribed.set(true) }}
       }
     }
   }