http: optimize headers iteration

This commit uses instanceof instead of Array.isArray() for faster
type checking and avoids calling Object.keys() when the headers are
stored as a 2D array instead of a plain object.

PR-URL: #6533
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Fedor Indutny <fedor.indutny@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>

lib/_http_outgoing.js

@@ -209,23 +209,31 @@ function _storeHeader(firstLine, headers) {
     messageHeader: firstLine
   };
 
-  if (headers) {
-    var keys = Object.keys(headers);
-    var isArray = Array.isArray(headers);
-    var field, value;
-
-    for (var i = 0, l = keys.length; i < l; i++) {
-      var key = keys[i];
-      if (isArray) {
-        field = headers[key][0];
-        value = headers[key][1];
+  var i;
+  var j;
+  var field;
+  var value;
+  if (headers instanceof Array) {
+    for (i = 0; i < headers.length; ++i) {
+      field = headers[i][0];
+      value = headers[i][1];
+
+      if (value instanceof Array) {
+        for (j = 0; j < value.length; j++) {
+          storeHeader(this, state, field, value[j]);
+        }
       } else {
-        field = key;
-        value = headers[key];
+        storeHeader(this, state, field, value);
       }
+    }
+  } else if (headers) {
+    var keys = Object.keys(headers);
+    for (i = 0; i < keys.length; ++i) {
+      field = keys[i];
+      value = headers[field];
 
-      if (Array.isArray(value)) {
-        for (var j = 0; j < value.length; j++) {
+      if (value instanceof Array) {
+        for (j = 0; j < value.length; j++) {
           storeHeader(this, state, field, value[j]);
         }
       } else {