always produce valid urls with trailing /

Fixes this error:

ERROR: connection error to wss://tracker.webtorrent.io?1fe16837ed
ERROR: If you think this is a bug in webtorrent, report it!

Author: feross

lib/websocket-tracker.js

@@ -25,7 +25,11 @@ function WebSocketTracker (client, announceUrl, opts) {
   self.destroyed = false
 
   self._opts = opts
+
   self._announceUrl = announceUrl
+  if (self._announceUrl[self._announceUrl.length - 1] !== '/') self._announceUrl += '/'
+  self._announceUrl += '?' + hat(40)
+
   self._peers = {} // peers (offer id -> peer)
   self._socket = null
   self._intervalMs = self.client._intervalMs // use client interval initially
@@ -98,7 +102,7 @@ WebSocketTracker.prototype._openSocket = function () {
   self._onSocketDataBound = self._onSocketData.bind(self)
   self._onSocketCloseBound = self._onSocketClose.bind(self)
 
-  self._socket = new Socket(self._announceUrl + '?' + hat(40))
+  self._socket = new Socket(self._announceUrl)
   self._socket.on('data', self._onSocketDataBound)
   self._socket.on('close', self._onSocketCloseBound)
   self._socket.on('error', self._onSocketErrorBound)