isBoolean addition

Change-Id: I83d428702314e6cd81f35b8bbbd04eed597cc04d

@@ -459,6 +459,19 @@ public abstract class Config<S> {
     }
 
     /**
+     * Indicates whether the specified field holds a valid boolean value.
+     *
+     * @param field    JSON field name
+     * @param presence specifies if field is optional or mandatory
+     * @return true if valid; false otherwise
+     * @throws IllegalArgumentException if field is present, but not valid
+     */
+    protected boolean isBoolean(String field, FieldPresence presence) {
+        JsonNode node = object.path(field);
+        return isValid(node, presence, node.isBoolean());
+    }
+
+    /**
      * Indicates whether the node is present and of correct value or not
      * mandatory and absent.
      *
@@ -471,5 +484,4 @@ public abstract class Config<S> {
         boolean isMandatory = presence == FieldPresence.MANDATORY;
         return isMandatory && correctValue || !isMandatory && !node.isNull() || correctValue;
     }
-
 }