[Timestampable] Allow multiple fields for "change" trigger in XML mapping

[bartosz-zasada]

According to the documentation, it's possible to set up multiple fields to trigger the "change" action. This, however, is only possible when using attribute mapping. As an XML mapping user, this feature is not available for me. I would greatly appreciate being able to take advantage of this feature without having to rewrite my entire entity mapping to attributes.

A one-liner change in \Gedmo\Timestampable\Mapping\Driver\Xml would allow a simple comma-separated list to be provided in the field attribute of the gedmo:timestampable element.