activitypub: transmogrifier: do not attempt to expand pre-existing AS2 tag objects

lib/pleroma/web/activity_pub/transmogrifier.ex

@@ -765,12 +765,18 @@ def maybe_fix_object_url(data) do
   def add_hashtags(object) do
     tags =
       (object["tag"] || [])
-      |> Enum.map(fn tag ->
+      |> Enum.map(fn
+        # Expand internal representation tags into AS2 tags.
+        tag when is_binary(tag) ->
           %{
             "href" => Pleroma.Web.Endpoint.url() <> "/tags/#{tag}",
             "name" => "##{tag}",
             "type" => "Hashtag"
           }
+
+        # Do not process tags which are already AS2 tag objects.
+        tag when is_map(tag) ->
+          tag
       end)
 
     object