fix #4 gracefully shutdown all ExecutorServices

src/test/java/com/github/nkzawa/engineio/client/Connection.java

@@ -65,8 +65,8 @@ public abstract class Connection {
     public void stopServer() throws InterruptedException {
         System.out.println("Stopping server ...");
         serverProcess.destroy();
-        serverOutout.cancel(true);
-        serverError.cancel(true);
+        serverOutout.cancel(false);
+        serverError.cancel(false);
         serverService.shutdown();
         serverService.awaitTermination(3000, TimeUnit.MILLISECONDS);
     }