Ensure the test SingleItemObservable emits item only once (#631)

Author: gael-ft

driver-scala/src/main/scala/org/mongodb/scala/internal/SingleItemObservable.scala

@@ -25,15 +25,18 @@ private[scala] case class SingleItemObservable[A](item: A) extends SingleObserva
       new Subscription {
         @volatile
         private var subscribed: Boolean = true
+        @volatile
+        private var completed: Boolean = false
 
         override def isUnsubscribed: Boolean = !subscribed
 
         override def request(n: Long): Unit = {
           require(n > 0L, s"Number requested must be greater than zero: $n")
 
-          if (subscribed) {
+          if (subscribed && !completed) {
             observer.onNext(item)
             observer.onComplete()
+            completed = true
           }
         }