Fix re-setting the close_status to None or reset it when re-opening a ticket
    
    This commit allows to re-set the close_status to nothing (its default
    value when the ticket is open), it also does so automatically when the
    status of a ticket is set to 'Open' (ie: the ticket is re-opened).
    
    Fixes : https://pagure.io/pagure/issue/1526