Fix compilation against the latest version of ndn-cxx

Remove Buffer.buf()

Change-Id: I9a460dd43e0b96220d3d44a63607408afa1b2473
diff --git a/tests/identity-management-fixture.cpp b/tests/identity-management-fixture.cpp
index f2b7e1b..1eef608 100644
--- a/tests/identity-management-fixture.cpp
+++ b/tests/identity-management-fixture.cpp
@@ -99,7 +99,7 @@
   certificate.setFreshnessPeriod(time::hours(1));
 
   // set content
-  certificate.setContent(key.getPublicKey().buf(), key.getPublicKey().size());
+  certificate.setContent(key.getPublicKey().get<uint8_t>(), key.getPublicKey().size());
 
   // set signature-info
   SignatureInfo info;