Dates of the sent activities are serialized as timestamps instead of ISO-8601 strings

Problem

Once accepted by Mastodon, a message sent from XWiki is never displayed on the Mastodon side: Mastodon fails while processing the received activity:

undefined method 'to_datetime' for an instance of Integer (NoMethodError)
    datetime = @object['published']&.to_datetime

Cause

Dates (published, updated...) are serialized as epoch milliseconds, e.g. "published" : 1785046974031. The ActivityStreams vocabulary types them as xsd:dateTime, so receivers expect an ISO-8601 string such as "published" : "2026-07-26T06:33:47.000+00:00".

Fix

Serialize the dates as ISO-8601 strings. Dates given as epoch milliseconds are still accepted when parsing, so the entities already stored in the old format remain readable.

Reproduced with Mastodon 4.6.3.