Fix token auth when service is missing

Make the `service` parameter optional when
fetching a bearer token, since some registries do
not use it.

The same logic can be found in containerd:
https://github.com/containerd/containerd/blob/61f91b963ef244daec1bda6700fe3f0b1aee50c6/core/remotes/docker/auth/fetch.go#L187

Author: jvstme

dxf/__init__.py

@@ -302,7 +302,8 @@ def authenticate(self,
                 scope = ''
             url_parts = list(urlparse.urlparse(info['realm']))
             query = urlparse.parse_qsl(url_parts[4])
-            query.append(('service', info['service']))
+            if 'service' in info:
+                query.append(('service', info['service']))
             query.extend(('scope', s) for s in scope.split())
             url_parts[4] = urlencode(query, True)
             url_parts[0] = 'https'