Support more variations of ActivityPub keyId in signature (#4630)

- Tries to avoid performing HTTP request if the keyId is an actor URI
- Likewise if the URI is a fragment URI on top of actor URI
- Resolves public key, returns owner if the owner links back to the key

app/lib/activitypub/activity/undo.rb

@@ -64,6 +64,6 @@ class ActivityPub::Activity::Undo < ActivityPub::Activity
   end
 
   def target_uri
-    @target_uri ||= @object['object'].is_a?(String) ? @object['object'] : @object['object']['id']
+    @target_uri ||= value_or_id(@object['object'])
   end
 end