b2: fix multipart upload retries #1733
b2: fix multipart upload retries #1733

Prior to this fix we were uploading 0 length bodies if a retry was
needed on a multipart upload chunk.  This gave this error `http:
ContentLength=268435496 with Body length 0`.

Fix by remaking the hash appending reader in the Call loop.  This is
inefficient in the face of retries, but these are uncommon.

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