fs: fix --immutable tests on remotes which don't have modtime
fs: fix --immutable tests on remotes which don't have modtime

yonjah/rclone,yonjah/rclone,ncw/rclone,yonjah/rclone,olihey/rclone,B4dM4n/rclone,ncw/rclone,seuffert/rclone,B4dM4n/rclone,B4dM4n/rclone,seuffert/rclone,ncw/rclone,B4dM4n/rclone,olihey/rclone,olihey/rclone,seuffert/rclone,ncw/rclone,seuffert/rclone,yonjah/rclone,olihey/rclone