torrent server only call internal `server.close` once

If the user calls `server.close()` on the http server returned by
`torrnet.createServer()` then we should not call it in
`server.destroy()` or node will return an error

lib/server.js

@@ -12,6 +12,7 @@ function Server (torrent, opts) {
   var server = http.createServer(opts)
 
   var sockets = []
+  var closed = false
 
   server.on('connection', function (socket) {
     socket.setTimeout(36000000)
@@ -22,11 +23,20 @@ function Server (torrent, opts) {
     })
   })
 
+  var _close = server.close
+  server.close = function (cb) {
+    closed = true
+    _close.call(server, cb)
+  }
+
   server.destroy = function (cb) {
     sockets.forEach(function (socket) {
       socket.destroy()
     })
-    server.close(cb)
+
+    // Only call `server.close` if user has not called it already
+    if (closed) process.nextTick(cb)
+    else server.close(cb)
   }
 
   server.on('request', function (req, res) {