Merge pull request #1729 from huangdx0726/fix1713

modifier field to final

Author: kazuki43zoo

src/main/java/org/apache/ibatis/cache/impl/PerpetualCache.java

@@ -28,7 +28,7 @@ public class PerpetualCache implements Cache {
 
   private final String id;
 
-  private Map<Object, Object> cache = new HashMap<>();
+  private final Map<Object, Object> cache = new HashMap<>();
 
   public PerpetualCache(String id) {
     this.id = id;