component AP gateway: we need to use actor and not attributedTo for activities:

`attributedTo` may not be filled, notably when the object is a string URL

sat/plugins/plugin_comp_ap_gateway/__init__.py

@@ -970,7 +970,7 @@ class APGateway:
                         comment_node = comment_data["node"]
                         await self._p.subscribe(client, comment_service, comment_node)
                 ap_item = await self.mbdata2APitem(client, mb_data)
-                url_actor = ap_item["object"]["attributedTo"]
+                url_actor = ap_item["actor"]
             elif item.name == "retract":
                 url_actor, ap_item = await self.apDeleteItem(
                     client.jid, node, item["id"]
@@ -1690,7 +1690,7 @@ class APGateway:
             raise exceptions.DataError("Can't get ActivityPub actor inbox")
 
         item_data = await self.mbdata2APitem(client, mess_data)
-        url_actor = item_data["object"]["attributedTo"]
+        url_actor = item_data["actor"]
         resp = await self.signAndPost(inbox_url, url_actor, item_data)
         if resp.code != 202:
             raise exceptions.NetworkError(f"unexpected return code: {resp.code}")