[fix] Replace deprecated Buffer usage (#565)

The `Buffer` constructor has been deprecated in favor of safer alternatives.
See https://nodejs.org/en/docs/guides/buffer-constructor-deprecation/

Author: oliversalzburg

README.md

@@ -20,7 +20,7 @@ var server = engine.listen(80);
 
 server.on('connection', function(socket){
   socket.send('utf 8 string');
-  socket.send(new Buffer([0, 1, 2, 3, 4, 5])); // binary data
+  socket.send(Buffer.from([0, 1, 2, 3, 4, 5])); // binary data
 });
 ```
 

lib/server.js

@@ -359,8 +359,7 @@ Server.prototype.handleUpgrade = function (req, socket, upgradeHead) {
       return;
     }
 
-    var head = new Buffer(upgradeHead.length); // eslint-disable-line node/no-deprecated-api
-    upgradeHead.copy(head);
+    var head = Buffer.from(upgradeHead); // eslint-disable-line node/no-deprecated-api
     upgradeHead = null;
 
     // delegate to ws