Fix executor used with AbstractCompletableTask.whenCompleteAsync

Author: vsilaev

src/main/java/net/tascalate/concurrent/AbstractCompletableTask.java

@@ -237,7 +237,7 @@ public Promise<T> exceptionally(Function<Throwable, ? extends T> fn) {
 
     @Override
     public Promise<T> whenCompleteAsync(BiConsumer<? super T, ? super Throwable> action, Executor executor) {
-        AbstractCompletableTask<T> nextStage = internalCreateCompletionStage(getDefaultExecutor());
+        AbstractCompletableTask<T> nextStage = internalCreateCompletionStage(executor);
         addCallbacks(
             nextStage, 
             result -> {