Code cleanup (#1461)

* Code cleanup

* Fix flux tests

* Fix checkstyle errors

* Fix compile error

poison-pill/src/main/java/com/iluwatar/poison/pill/SimpleMessage.java

@@ -32,7 +32,7 @@ import java.util.Map;
  */
 public class SimpleMessage implements Message {
 
-  private Map<Headers, String> headers = new HashMap<>();
+  private final Map<Headers, String> headers = new HashMap<>();
   private String body;
 
   @Override

poison-pill/src/test/java/com/iluwatar/poison/pill/ConsumerTest.java

@@ -92,7 +92,7 @@ public class ConsumerTest {
   }
 
   private class InMemoryAppender extends AppenderBase<ILoggingEvent> {
-    private List<ILoggingEvent> log = new LinkedList<>();
+    private final List<ILoggingEvent> log = new LinkedList<>();
 
     public InMemoryAppender(Class clazz) {
       ((Logger) LoggerFactory.getLogger(clazz)).addAppender(this);