config ang key validation and UT for the same

Change-Id: I507740fc9da3f3da5fb3c88a7414f87db6251c5b
diff --git a/src/main/java/org/onosproject/yangutils/datamodel/YangLeafList.java b/src/main/java/org/onosproject/yangutils/datamodel/YangLeafList.java
index 18653d2..6ce3430 100644
--- a/src/main/java/org/onosproject/yangutils/datamodel/YangLeafList.java
+++ b/src/main/java/org/onosproject/yangutils/datamodel/YangLeafList.java
@@ -62,7 +62,7 @@
     /**
      * If the leaf-list is a config parameter.
      */
-    private boolean isConfig;
+    private Boolean isConfig;
 
     /**
      * Description of leaf-list.
@@ -150,7 +150,7 @@
      *
      * @return the config flag.
      */
-    public boolean isConfig() {
+    public Boolean isConfig() {
         return isConfig;
     }