fix(Error.custom): improve ext argument detection

Author: medikoo

error/custom.js

@@ -1,13 +1,14 @@
 'use strict';
 
-var assign = require('../object/assign')
+var assign   = require('../object/assign')
+  , isObject = require('../object/is-object')
 
   , captureStackTrace = Error.captureStackTrace;
 
 exports = module.exports = function (message/*, code, ext*/) {
 	var err = new Error(message), code = arguments[1], ext = arguments[2];
 	if (ext == null) {
-		if (code && (typeof code === 'object')) {
+		if (isObject(code)) {
 			ext = code;
 			code = null;
 		}