security: Make addKey/Cert behavior of PibImpl consistent

Change-Id: I30c6dd0007e9095ee07f7a8eda3b20db4c34c513
Refs: #3351
diff --git a/src/security/v2/certificate.cpp b/src/security/v2/certificate.cpp
index b5188e9..b6a08ab 100644
--- a/src/security/v2/certificate.cpp
+++ b/src/security/v2/certificate.cpp
@@ -98,7 +98,7 @@
   return getName().at(ISSUER_ID_OFFSET);
 }
 
-const Buffer
+Buffer
 Certificate::getPublicKey() const
 {
   if (getContent().value_size() == 0)