Populate device field on episodes_actions if the device is present in the request

[manuel-rabade]

While testing the API in the latest oPodSync update, I found that the device field in episodes_actions is not being populated for new actions. The old actions were correctly migrated, and the device field is populated for them.

Please let me know if this change makes sense or if you have any feedback.

Looking forward to contributing to this nice and useful project. Thanks a lot!