Set user-agent header for http tracker requests

Fixes: #962

lib/torrent.js

@@ -52,6 +52,7 @@ var FILESYSTEM_CONCURRENCY = 2
 var RECONNECT_WAIT = [ 1000, 5000, 15000 ]
 
 var VERSION = require('../package.json').version
+var USER_AGENT = 'WebTorrent/' + VERSION + ' (https://webtorrent.io)'
 
 var TMP
 try {
@@ -332,7 +333,8 @@ Torrent.prototype._onListening = function () {
     peerId: self.client.peerId,
     dht: !self.private && self.client.dht,
     tracker: trackerOpts,
-    port: self.client.torrentPort
+    port: self.client.torrentPort,
+    userAgent: USER_AGENT
   })
 
   self.discovery.on('error', onError)
@@ -395,7 +397,7 @@ Torrent.prototype._getMetadataFromServer = function () {
       url: url,
       method: 'GET',
       headers: {
-        'user-agent': 'WebTorrent/' + VERSION + ' (https://webtorrent.io)'
+        'user-agent': USER_AGENT
       }
     }
     var req