b2: fix uploading empty files with go1.8
b2: fix uploading empty files with go1.8

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