face: pass processEvents duration by value

refs #3284

Change-Id: I146417f49c73fe93c6b38bb0dcd1b45bb949511f
diff --git a/src/util/dummy-client-face.hpp b/src/util/dummy-client-face.hpp
index 630913c..781a797 100644
--- a/src/util/dummy-client-face.hpp
+++ b/src/util/dummy-client-face.hpp
@@ -1,5 +1,5 @@
 /* -*- Mode:C++; c-file-style:"gnu"; indent-tabs-mode:nil; -*- */
-/**
+/*
  * Copyright (c) 2013-2017 Regents of the University of California.
  *
  * This file is part of ndn-cxx library (NDN C++ library with eXperimental eXtensions).
@@ -120,7 +120,7 @@
   enableRegistrationReply();
 
   void
-  doProcessEvents(const time::milliseconds& timeout, bool keepThread) override;
+  doProcessEvents(time::milliseconds timeout, bool keepThread) override;
 
 public:
   /** \brief Interests sent out of this DummyClientFace