Use public member instead of private field to resolve immutability fo…

…r serializer

src/com/esotericsoftware/kryo/Serializer.java

@@ -100,7 +100,7 @@ public void setGenerics (Kryo kryo, Class[] generics) {
 	 * This method should not be called directly, instead this serializer can be passed to {@link Kryo} copy methods that accept a
 	 * serialier. */
 	public T copy (Kryo kryo, T original) {
-		if (immutable) return original;
+		if (isImmutable()) return original;
 		throw new KryoException("Serializer does not support copy: " + getClass().getName());
 	}
 }