Fix createMultiServer in newer nodejs versions

Make sure to set `socket.server` to the appropriate server handler before delegating.

This became required as of node v9.6, due to the changes introduced in nodejs/node@c247cb0

Fixes #10.

src/transport/tls.js

@@ -119,6 +119,7 @@ const createMultiServer = (tlsOpt, tlsHandler, plainHandler) => {
       socket.unshift(buff)
 
       // Delegate the socket to the appropriate handler
+      socket.server = server[proto]
       server[proto].emit('connection', socket)
 
       socket.resume()