Use final modifier on fields of ConditionalSchema class.

Author: zgyorffi

core/src/main/java/org/everit/json/schema/ConditionalSchema.java

@@ -40,9 +40,9 @@ public static Builder builder() {
         return new Builder();
     }
 
-    private Schema ifSchema;
-    private Schema thenSchema;
-    private Schema elseSchema;
+    private final Schema ifSchema;
+    private final Schema thenSchema;
+    private final Schema elseSchema;
 
     public ConditionalSchema(Builder builder) {
         super(builder);