Use Array.isArray instead of instanceof

lib/http.js

@@ -359,7 +359,7 @@ OutgoingMessage.prototype._storeHeader = function (firstLine, headers) {
 
   if (headers) {
     var keys = Object.keys(headers);
-    var isArray = (headers instanceof Array);
+    var isArray = (Array.isArray(headers));
     for (var i = 0, l = keys.length; i < l; i++) {
       var key = keys[i];
       if (isArray) {
@@ -1036,7 +1036,7 @@ exports.cat = function (url, encoding_, headers_) {
   var url = require("url").parse(url);
 
   var hasHost = false;
-  if (headers instanceof Array) {
+  if (Array.isArray(headers)) {
     for (var i = 0, l = headers.length; i < l; i++) {
       if (headers[i][0].toLowerCase() === 'host') {
         hasHost = true;