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

Author: Connormiha

lib/stringify.js

@@ -111,7 +111,7 @@ var stringify = function stringify(
 
     for (var j = 0; j < objKeys.length; ++j) {
         var key = objKeys[j];
-        var value = typeof key === 'object' && key.value !== undefined ? key.value : obj[key];
+        var value = typeof key === 'object' && typeof key.value !== 'undefined' ? key.value : obj[key];
 
         if (skipNulls && obj[key] === null) {
             continue;
@@ -160,7 +160,7 @@ var normalizeStringifyOptions = function normalizeStringifyOptions(opts) {
         return defaults;
     }
 
-    if (opts.encoder !== null && opts.encoder !== undefined && typeof opts.encoder !== 'function') {
+    if (opts.encoder !== null && typeof opts.encoder !== 'undefined' && typeof opts.encoder !== 'function') {
         throw new TypeError('Encoder has to be a function.');
     }