Make JSON codecs immutable

The contract for JSON codecs requires that they have no
local context.  Make them all immutable, and test that they
stay immutable.

Change-Id: Ia3930b844ede231940da05699f5f08605f946e0c
diff --git a/web/api/src/main/java/org/onosproject/codec/impl/ApplicationCodec.java b/web/api/src/main/java/org/onosproject/codec/impl/ApplicationCodec.java
index 4cf0b7c..7a148de 100644
--- a/web/api/src/main/java/org/onosproject/codec/impl/ApplicationCodec.java
+++ b/web/api/src/main/java/org/onosproject/codec/impl/ApplicationCodec.java
@@ -26,7 +26,7 @@
 /**
  * Application JSON codec.
  */
-public class ApplicationCodec extends JsonCodec<Application> {
+public final class ApplicationCodec extends JsonCodec<Application> {
 
     @Override
     public ObjectNode encode(Application app, CodecContext context) {