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/InstructionCodec.java b/web/api/src/main/java/org/onosproject/codec/impl/InstructionCodec.java
index 29d1ad7..57f95bd 100644
--- a/web/api/src/main/java/org/onosproject/codec/impl/InstructionCodec.java
+++ b/web/api/src/main/java/org/onosproject/codec/impl/InstructionCodec.java
@@ -33,7 +33,7 @@
 /**
  * Instruction codec.
  */
-public class InstructionCodec extends JsonCodec<Instruction> {
+public final class InstructionCodec extends JsonCodec<Instruction> {
 
     protected static final Logger log = LoggerFactory.getLogger(InstructionCodec.class);