Fix validateType method call

Author: renonick87

lib/js/src/rpc/RpcResponse.js

@@ -95,7 +95,7 @@ class RpcResponse extends RpcMessage {
      * @returns {RpcResponse} - A reference of this class to support method chaining.
      */
     setResultCode (resultCode) {
-        this.validateType(Result, resultCode);
+        this._validateType(Result, resultCode);
 
         this.setParameter(RpcResponse.KEY_RESULT_CODE, resultCode);
         return this;