all: InterestHeader and ContentObjectHeader refactoring to Interest and ContentObject

refs #29
diff --git a/helper/ndn-header-helper.cc b/helper/ndn-header-helper.cc
index ee22b43..f9145cc 100644
--- a/helper/ndn-header-helper.cc
+++ b/helper/ndn-header-helper.cc
@@ -81,7 +81,7 @@
         {
         case HeaderHelper::INTEREST_NDNSIM:
           {
-            Ptr<InterestHeader> header = Create<InterestHeader> ();
+            Ptr<Interest> header = Create<Interest> ();
 
             // Deserialization. Exception may be thrown
             packet->RemoveHeader (*header);
@@ -92,7 +92,7 @@
           }
         case HeaderHelper::CONTENT_OBJECT_NDNSIM:
           {
-            Ptr<ContentObjectHeader> header = Create<ContentObjectHeader> ();
+            Ptr<ContentObject> header = Create<ContentObject> ();
 
             // Deserialization. Exception may be thrown
             packet->RemoveHeader (*header);