introduce a configuration option to define the internaldate used for APPEND

Hello, I'm using Offlineimap, my mail provider is FastMail. Whenever I'm moving a file around locally (from one Maildir directory into another one) the mail is first being deleted on the server, and then uploaded again, so far so good. But when the mail is uploaded again the internaldate of the message get's changed to the current timestamp:

2018-02-06 13:28:26 DEBUG: [imap]: savemessage: date: "06-Feb-2018 13:27:33 +0100", […]

So in the end the mail shows on FastMail with the wrong date. Correct woud be the date which is set in the Date header of the mail. How can I configure Offlineimap to respect the Date header here?