Public static field should be constant

Author: matteobaccan

core/src/main/java/pl/project13/core/log/FormattingTuple.java

@@ -49,7 +49,7 @@
  */
 public class FormattingTuple {
 
-  public static FormattingTuple NULL = new FormattingTuple(null);
+  public static final FormattingTuple NULL = new FormattingTuple(null);
 
   private String message;
   private Throwable throwable;