[Robustness] stringify: avoid relying on a global undefined (#427)

Author: Connormiha

lib/stringify.js

@@ -89,7 +89,7 @@ module.exports = function (object, opts) {
     var objKeys;
     var filter;
 
-    if (options.encoder !== null && options.encoder !== undefined && typeof options.encoder !== 'function') {
+    if (options.encoder !== null && typeof options.encoder !== 'undefined' && typeof options.encoder !== 'function') {
         throw new TypeError('Encoder has to be a function.');
     }