all: InterestHeader and ContentObjectHeader refactoring to Interest and ContentObject

refs #29
diff --git a/model/ndn-l3-protocol.cc b/model/ndn-l3-protocol.cc
index 28d337b..4719434 100644
--- a/model/ndn-l3-protocol.cc
+++ b/model/ndn-l3-protocol.cc
@@ -261,7 +261,7 @@
         case HeaderHelper::INTEREST_NDNSIM:
           {
             s_interestCounter ++;
-            Ptr<InterestHeader> header = Create<InterestHeader> ();
+            Ptr<Interest> header = Create<Interest> ();
 
             // Deserialization. Exception may be thrown
             packet->RemoveHeader (*header);
@@ -277,7 +277,7 @@
         case HeaderHelper::CONTENT_OBJECT_NDNSIM:
           {
             s_dataCounter ++;
-            Ptr<ContentObjectHeader> header = Create<ContentObjectHeader> ();
+            Ptr<ContentObject> header = Create<ContentObject> ();
 
             static ContentObjectTail contentObjectTrailer; //there is no data in this object