Fix jObj.hasOwnProperty is not a function (#613)

* Fix `jObj.hasOwnProperty is not a function`

* Update json2xml.js

Author: ardatan

Diff for: src/xmlbuilder/json2xml.js

@@ -79,7 +79,7 @@ Builder.prototype.j2x = function(jObj, level) {
   let attrStr = '';
   let val = '';
   for (let key in jObj) {
-    if(!jObj.hasOwnProperty(key)) continue;
+    if(!Object.prototype.hasOwnProperty.call(jObj, key)) continue;
     if (typeof jObj[key] === 'undefined') {
       // supress undefined node only if it is not an attribute
       if (this.isAttribute(key)) {