model: Change default behavior to cache unsolicited data from local applications

In many cases, when a real NDN application is written, it is desirable
to be able to push content to the local cache (e.g., "pre-publish").

http://redmine.named-data.net/projects/ndnsim refs #1002
diff --git a/model/ndn-app-face.cc b/model/ndn-app-face.cc
index 9f9265e..669f9a8 100644
--- a/model/ndn-app-face.cc
+++ b/model/ndn-app-face.cc
@@ -58,6 +58,7 @@
   NS_LOG_FUNCTION (this << app);
   
   NS_ASSERT (m_app != 0);
+  SetFlags (Face::APPLICATION);
 }
 
 AppFace::~AppFace ()