s3: preserve metadata when doing multipart copy
s3: preserve metadata when doing multipart copy

Before this change the s3 multipart server side copy was not
preserving the metadata of the object. This was most noticeable
because the modtime was not preserved.

This change fetches the metadata from the object before starting the
copy and overwrites it if requires.

It will also mean any other metadata is preserved.

See: https://forum.rclone.org/t/copying-files-within-a-b2-bucket/16680/70

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