http: fix bailout for writeHead

Reported-by: Jackson Tian <shyvo1987@gmail.com>
Reviewed-by: Trevor Norris <trev.norris@gmail.com>

lib/_http_server.js

@@ -172,21 +172,20 @@ ServerResponse.prototype._implicitHeader = function() {
   this.writeHead(this.statusCode);
 };
 
-ServerResponse.prototype.writeHead = function(statusCode) {
-  var headers, headerIndex;
+ServerResponse.prototype.writeHead = function(statusCode, reason, obj) {
+  var headers;
 
-  if (util.isString(arguments[1])) {
-    this.statusMessage = arguments[1];
-    headerIndex = 2;
+  if (util.isString(reason)) {
+    // writeHead(statusCode, reasonPhrase[, headers])
+    this.statusMessage = reason;
   } else {
+    // writeHead(statusCode[, headers])
     this.statusMessage =
         this.statusMessage || STATUS_CODES[statusCode] || 'unknown';
-    headerIndex = 1;
+    obj = reason;
   }
   this.statusCode = statusCode;
 
-  var obj = arguments[headerIndex];
-
   if (this._headers) {
     // Slow-case: when progressive API and header fields are passed.
     if (obj) {