// about HTTP headers, Set should be used instead of
// http.Header.Set(...). Otherwise, some S3 clients will not
// able to extract the ETag.
func Set(etag ETag, h http.Header) {
	// Some (broken) S3 clients expect the ETag header to
	// literally "ETag" - not "Etag". Further, some clients
	// expect an ETag in double quotes. Therefore, we set the
	// ETag directly as map entry instead of using http.Header.Set