Fix problem serializing objects with null prototype

Author: sabberworm

src/index.js

@@ -25,7 +25,7 @@ var param = function (a) {
                         obj[i]
                     );
                 }
-            } else if (String(obj) === '[object Object]') {
+            } else if (Object.prototype.toString.call(obj) === '[object Object]') {
                 for (key in obj) {
                     buildParams(prefix + '[' + key + ']', obj[key]);
                 }