Add published property to ActivityPub activity for reblogs (#5000)

Since reblogs are serialized as Announce activity, its published property can be used for the creation time of reblog.

app/lib/activitypub/activity/announce.rb

@@ -11,7 +11,12 @@ class ActivityPub::Activity::Announce < ActivityPub::Activity
 
     return status unless status.nil?
 
-    status = Status.create!(account: @account, reblog: original_status, uri: @json['id'])
+    status = Status.create!(
+      account: @account,
+      reblog: original_status,
+      uri: @json['id'],
+      created_at: @json['published'] || Time.now.utc
+    )
     distribute(status)
     status
   end

app/serializers/activitypub/activity_serializer.rb

 # frozen_string_literal: true
 
 class ActivityPub::ActivitySerializer < ActiveModel::Serializer
-  attributes :id, :type, :actor, :to, :cc
+  attributes :id, :type, :actor, :published, :to, :cc
 
   has_one :proper, key: :object, serializer: ActivityPub::NoteSerializer
 
@@ -17,6 +17,10 @@ class ActivityPub::ActivitySerializer < ActiveModel::Serializer
     ActivityPub::TagManager.instance.uri_for(object.account)
   end
 
+  def published
+    object.created_at.iso8601
+  end
+
   def to
     ActivityPub::TagManager.instance.to(object)
   end