[ndnSIM] face: Emulation mode of ndn::Face

Change-Id: Ib345978575104afa436296d2f5a824a39e3a9780
diff --git a/src/detail/pending-interest.hpp b/src/detail/pending-interest.hpp
index 41f6b37..9cea52c 100644
--- a/src/detail/pending-interest.hpp
+++ b/src/detail/pending-interest.hpp
@@ -76,9 +76,9 @@
    * @note If the DataCallback is an empty function, this method does nothing.
    */
   void
-  invokeDataCallback(Data& data)
+  invokeDataCallback(const Data& data)
   {
-    m_onData(*m_interest, data);
+    m_onData(*m_interest, const_cast<Data&>(data));
   }
 
   /**