question_validator: fix for mastodon poll expiration

Mastodon activities do not have a "closed" field, this could be seen on
https://pouet.it/users/lanodan_tmp/statuses/104345126997708380
which runs Mastodon 3.1.4 (SDF runs 3.1.2)

lib/pleroma/web/activity_pub/object_validators/question_validator.ex

@@ -71,7 +71,17 @@ defmodule Pleroma.Web.ActivityPub.ObjectValidators.QuestionValidator do
     |> changeset(data)
   end
 
+  def fix(data) do
+    cond do
+      is_binary(data["closed"]) -> data
+      is_binary(data["endTime"]) -> Map.put(data, "closed", data["endTime"])
+      true -> Map.drop(data, ["closed"])
+    end
+  end
+
   def changeset(struct, data) do
+    data = fix(data)
+
     struct
     |> cast(data, __schema__(:fields) -- [:anyOf, :oneOf])
     |> cast_embed(:anyOf)