Use created or modified timestamp as fallback (#22)

            }

            let file_meta = try_!(entry.metadata().await);

            let creation_date = Timestamp::from(try_!(file_meta.created()));

            // Not all filesystems/mounts provide all file attributes like created timestamp,

            // therefore we try to fallback to modified if possible.

            // See https://github.com/Nugine/s3s/pull/22 for more details.

            let created_or_modified_date = Timestamp::from(try_!(file_meta.created().or(file_meta.modified())));

            let bucket = Bucket {

                creation_date: Some(creation_date),

                creation_date: Some(created_or_modified_date),

                name: Some(name.to_owned()),

            };

            buckets.push(bucket);