Send Content-MD5 header on S3 uploads (#728)

Set SendContentMd5: true in PutObjectOptions so that minio-go
includes a Content-MD5 header with every PutObject request.

AWS S3 requires this header when the target bucket has Object Lock
enabled; without it the upload is rejected with HTTP 400
(InvalidRequest).  The overhead is negligible — one MD5 hash per
upload — and the header also serves as a server-side integrity
check for all uploads regardless of Object Lock.

Fixes #727

internal/storage/s3/s3.go

@@ -107,6 +107,7 @@ func (b *s3Storage) Copy(file string) error {
 	putObjectOptions := minio.PutObjectOptions{
 		ContentType:    "application/tar+gzip",
 		StorageClass:   b.storageClass,
+		SendContentMd5: true,
 	}
 
 	if b.partSize > 0 {