fix: Prototype fields conflict with dom attributes

As noted in issue xmldom#252 some attributes conflict with prototype fields (such as an attribute `constructor`) thus we must remove the `in` operator and replace with `Object.prototype.hasOwnProperty()`.

lib/sax.js

@@ -230,7 +230,7 @@ function parseElementStartPart(source,start,el,currentNSMap,entityReplacer,error
 	 * @param {number} startIndex
 	 */
 	function addAttribute(qname, value, startIndex) {
-		if (qname in el.attributeNames) errorHandler.fatalError('Attribute ' + qname + ' redefined')
+		if (el.attributeNames.hasOwnProperty(qname)) errorHandler.fatalError('Attribute ' + qname + ' redefined')
 		el.addValue(qname, value, startIndex)
 	}
 	var attrName;