Fix error with unicode URLs in torrent creation

Fixes https://jira.coreos.com/browse/QUAY-896

util/registry/test/test_torrent.py

@@ -0,0 +1,4 @@
+from util.registry.torrent import make_torrent
+
+def test_make_torrent_unicode_url():
+  make_torrent('foo', unicode('bar'), 10, 20, 'hello world')

util/registry/torrent.py

@@ -55,7 +55,7 @@ def make_torrent(name, webseed, length, piece_length, pieces):
 
   return bencode.bencode({
     'announce': ANNOUNCE_URL + "?jwt=" + jwt_from_infodict(info_dict),
-    'url-list': webseed,
+    'url-list': str(webseed),
     'encoding': 'UTF-8',
     'created by': REGISTRY_TITLE,
     'creation date': int(time.time()),