Don't wrap ExecutionException again when completing a CompletableFuture, close AsyncHttpClient#1163

Author: slandelle

client/src/main/java/org/asynchttpclient/netty/NettyResponseFuture.java

@@ -266,7 +266,7 @@ public CompletableFuture<V> toCompletableFuture() {
             public void run() {
                 ExecutionException e = EX_EX_UPDATER.get(NettyResponseFuture.this);
                 if (e != null)
-                    completable.completeExceptionally(e);
+                    completable.completeExceptionally(e.getCause());
                 else
                     completable.complete((V) CONTENT_UPDATER.get(NettyResponseFuture.this));
             }