[fix] Add proper response when handleUpgrade fails (#458)

When the `verify` method fails, the current implementation closes the
connection immediately, which is understood by some proxy (such as
nginx) as if the server was not available (resulting in "upstream
prematurely closed connection while reading response header from
upstream" error). That commit make sure a proper response is sent
before closing the connection.

lib/server.js

@@ -321,8 +321,8 @@ Server.prototype.handleUpgrade = function (req, socket, upgradeHead) {
 
   var self = this;
   this.verify(req, true, function (err, success) {
-    if (err) {
-      socket.end();
+    if (!success) {
+      abortConnection(socket, err);
       return;
     }
 
@@ -455,3 +455,27 @@ Server.prototype.attach = function (server, options) {
     });
   }
 };
+
+/**
+ * Closes the connection
+ *
+ * @param {net.Socket} socket
+ * @param {code} error code
+ * @api private
+ */
+
+function abortConnection (socket, code) {
+  if (socket.writable) {
+    var message = Server.errorMessages.hasOwnProperty(code) ? Server.errorMessages[code] : code;
+    var length = Buffer.byteLength(message);
+    socket.write(
+      'HTTP/1.1 400 Bad Request\r\n' +
+      'Connection: close\r\n' +
+      'Content-type: text/html\r\n' +
+      'Content-Length: ' + length + '\r\n' +
+      '\r\n' +
+      message
+    );
+  }
+  socket.destroy();
+}