description changes are lost if user changes the due date

[jakobroehrl]

@juliushaertl
If the user changes the description and changes the due all changes made to the description are lost after page reload.
If I only change the description all changes are saved.
Tested on try.nextcloud, too.

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We save here the due date with the complete copied card. But we don't know if the description was changed, because this happens in another component:
https://github.com/nextcloud/deck/blob/master/src/components/card/CardSidebarTabDetails.vue#L216-L217 (update)
https://github.com/nextcloud/deck/blob/master/src/components/card/CardSidebarTabDetails.vue#L120 (description)