Minor fixes in DatabaseProxy

Change-Id: I2e1ac92a80f652acc324337383dba526e510f353
diff --git a/core/store/serializers/src/main/java/org/onosproject/store/serializers/KryoNamespaces.java b/core/store/serializers/src/main/java/org/onosproject/store/serializers/KryoNamespaces.java
index 2f34408..6784ab3 100644
--- a/core/store/serializers/src/main/java/org/onosproject/store/serializers/KryoNamespaces.java
+++ b/core/store/serializers/src/main/java/org/onosproject/store/serializers/KryoNamespaces.java
@@ -18,6 +18,7 @@
 import com.google.common.collect.ImmutableList;
 import com.google.common.collect.ImmutableMap;
 import com.google.common.collect.ImmutableSet;
+import com.google.common.collect.Maps;
 
 import org.onlab.packet.ChassisId;
 import org.onlab.packet.EthType;
@@ -222,6 +223,7 @@
                       LinkedList.class,
                       HashSet.class
                       )
+            .register(Maps.immutableEntry("a", "b").getClass())
             .register(new ArraysAsListSerializer(), Arrays.asList().getClass())
             .register(Collections.singletonList(1).getClass())
             .register(Duration.class)