activitypub.postprocess_as2: use real AP actor id in key id

activitypub.py

@@ -551,7 +551,7 @@ def postprocess_as2(activity, orig_obj=None, wrap=True):
             # underspecified, inferred from this issue and Mastodon's implementation:
             # https://github.com/w3c/activitypub/issues/203#issuecomment-297553229
             # https://github.com/tootsuite/mastodon/blob/bc2c263504e584e154384ecc2d804aeb1afb1ba3/app/services/activitypub/process_account_service.rb#L77
-            actor_url = host_url(activity.get('preferredUsername'))
+            actor_url = g.user.ap_actor()
             activity.update({
                 'publicKey': {
                     'id': f'{actor_url}#key',