b2: Fix reading metadata for all files when using a subdir - fixes #356
b2: Fix reading metadata for all files when using a subdir - fixes #356

Also fix some confusion with Metadata prefix/root.

jackschmidt/rclone,B4dM4n/rclone,ncw/rclone,yonjah/rclone,arcimboldo/rclone,kynikos/rclone,jackschmidt/rclone,arcimboldo/rclone,olihey/rclone,B4dM4n/rclone,arcimboldo/rclone,yonjah/rclone,seuffert/rclone,seuffert/rclone,ncw/rclone,kynikos/rclone,jackschmidt/rclone,yonjah/rclone,yonjah/rclone,olihey/rclone,kynikos/rclone,olihey/rclone,ncw/rclone,olihey/rclone,B4dM4n/rclone,seuffert/rclone,ncw/rclone,jackschmidt/rclone,arcimboldo/rclone,seuffert/rclone,B4dM4n/rclone,kynikos/rclone