[livestream] Correct playlist ID and add a test for it

youtube_dl/extractor/livestream.py

@@ -37,6 +37,7 @@ class LivestreamIE(InfoExtractor):
         'url': 'http://new.livestream.com/tedx/cityenglish',
         'info_dict': {
             'title': 'TEDCity2.0 (English)',
+            'id': '2245590',
         },
         'playlist_mincount': 4,
     }, {
@@ -148,7 +149,8 @@ class LivestreamIE(InfoExtractor):
                   if is_relevant(video_data, video_id)]
         if video_id is None:
             # This is an event page:
-            return self.playlist_result(videos, info['id'], info['full_name'])
+            return self.playlist_result(
+                videos, '%s' % info['id'], info['full_name'])
         else:
             if not videos:
                 raise ExtractorError('Cannot find video %s' % video_id)