Code cleanup (#1461)

* Code cleanup

* Fix flux tests

* Fix checkstyle errors

* Fix compile error

chain/README.md

@@ -65,7 +65,7 @@ Then the request handler hierarchy
 ```java
 public abstract class RequestHandler {
   private static final Logger LOGGER = LoggerFactory.getLogger(RequestHandler.class);
-  private RequestHandler next;
+  private final RequestHandler next;
 
   public RequestHandler(RequestHandler next) {
     this.next = next;

chain/src/main/java/com/iluwatar/chain/RequestHandler.java

@@ -33,7 +33,7 @@ public abstract class RequestHandler {
 
   private static final Logger LOGGER = LoggerFactory.getLogger(RequestHandler.class);
 
-  private RequestHandler next;
+  private final RequestHandler next;
 
   public RequestHandler(RequestHandler next) {
     this.next = next;