b2: force the case of the SHA1 to lowercase - fixes #4162
b2: force the case of the SHA1 to lowercase - fixes #4162

Apparently some tools (eg duplicati) upload the SHA1 in uppercase to
b2 to be stored in the `large_file_sha1` metadata. This patch forces
it to lower case.

ncw/rclone,ncw/rclone,ncw/rclone,ncw/rclone