data: Extensions to simplify coding

Change-Id: I69a123e2d2770127ad5e7d0a8a5e5ef714d5da4f
diff --git a/include/ndn-cpp/data.hpp b/include/ndn-cpp/data.hpp
index 52b3c89..e2d9451 100644
--- a/include/ndn-cpp/data.hpp
+++ b/include/ndn-cpp/data.hpp
@@ -146,6 +146,7 @@
 
 inline
 Data::Data()
+  : content_(Tlv::Content) // empty content
 {
 }