BUG: Default value of ns3 object system is created exactly once, not per CreateObject operation.

Previous code caused interesting behavior because there was exactly one Limits object for all faces for all nodes.
diff --git a/model/ndn-face.cc b/model/ndn-face.cc
index 9b5bc89..d709cfc 100644
--- a/model/ndn-face.cc
+++ b/model/ndn-face.cc
@@ -56,7 +56,8 @@
                    MakeUintegerChecker<uint32_t> ())
 
     .AddAttribute ("Limits", "Limits object",
-                   PointerValue (CreateObject<Limits> ()),
+                   TypeId::ATTR_GET, // allow only getting it.
+                   PointerValue (0), // this is not really used, but needed for the compiler... so sad
                    MakePointerAccessor (&Face::m_limits),
                    MakePointerChecker<Limits> ())
 
@@ -86,6 +87,7 @@
   , m_ifup (false)
   , m_id ((uint32_t)-1)
   , m_metric (0)
+  , m_limits (CreateObject<Limits> ())
   // , m_enableMetricTagging (false)
 {
   NS_LOG_FUNCTION (this);