squash! Expose expires_at datetime in mastoAPI only for the activity actor

NOTE: rewrite the commit msg

test/web/mastodon_api/mastodon_api_controller_test.exs

@@ -170,7 +170,7 @@ defmodule Pleroma.Web.MastodonAPI.MastodonAPIControllerTest do
       assert activity = Activity.get_by_id(fourth_id)
       assert expiration = ActivityExpiration.get_by_activity_id(fourth_id)
       assert expiration.scheduled_at == expires_at
-      assert fourth_response["pleroma"]["expires_at"] == NaiveDateTime.to_iso8601(expires_at)
+      assert fourth_response["pleroma"]["expires_in"] > 0
     end
 
     test "replying to a status", %{conn: conn} do

test/web/mastodon_api/status_view_test.exs

@@ -134,7 +134,7 @@ defmodule Pleroma.Web.MastodonAPI.StatusViewTest do
         in_reply_to_account_acct: nil,
         content: %{"text/plain" => HtmlSanitizeEx.strip_tags(object_data["content"])},
         spoiler_text: %{"text/plain" => HtmlSanitizeEx.strip_tags(object_data["summary"])},
-        expires_at: nil
+        expires_in: nil
       }
     }