Fix 500 on get label endpoint and add a test

Fixes #2133

test/test_api_usage.py

@@ -4302,6 +4302,14 @@ class TestRepositoryManifestLabels(ApiTestCase):
     self.assertEquals('text/plain', label2['label']['media_type'])
     self.assertEquals('application/json', label3['label']['media_type'])
 
+    # Ensure we can retrieve each of the labels.
+    for label in json['labels']:
+      label_json = self.getJsonResponse(ManageRepositoryManifestLabel,
+                                        params=dict(repository=repository,
+                                                    manifestref=tag_manifest.digest,
+                                                    labelid=label['id']))
+      self.assertEquals(label['id'], label_json['id'])
+
     # Delete a label.
     with assert_action_logged('manifest_label_delete'):
       self.deleteResponse(ManageRepositoryManifestLabel,