security: Make addKey/Cert behavior of PibImpl consistent

Change-Id: I30c6dd0007e9095ee07f7a8eda3b20db4c34c513
Refs: #3351
diff --git a/src/security/pib/pib-memory.hpp b/src/security/pib/pib-memory.hpp
index c48f9fc..192f000 100644
--- a/src/security/pib/pib-memory.hpp
+++ b/src/security/pib/pib-memory.hpp
@@ -132,13 +132,13 @@
   std::set<Name> m_identities;
 
   /// @brief identity => default key Name
-  std::map<Name, Name> m_defaultKey;
+  std::map<Name, Name> m_defaultKeys;
 
   /// @brief keyName => keyBits
   std::map<Name, Buffer> m_keys;
 
   /// @brief keyName => default certificate Name
-  std::map<Name, Name> m_defaultCert;
+  std::map<Name, Name> m_defaultCerts;
 
   /// @brief certificate Name => certificate
   std::map<Name, v2::Certificate> m_certs;