Modified "historicState" to return HistoricItem as per other extensions

As mentioned in the other request, whenever I upload the persistence extension file to github, it shows all lines changed - locally I don't see this. Maybe there's something "hidden" in the file (line terminations or something) that github doesn't like?

This is a new branch taken from the latest openhab master (with the latest license information) and GitHub does say it is able to merge automatically so hopefully it's ok.