Fix PutMetadata with S3 backend (#176)

It turns out that the S3 API expects the additional `MetadataDirective:
REPLACE` option in order to update metadata. If this is not provided,
then metadata will simply be copied from the source object, which is not
what we wanted.

backends/s3/s3.go

@@ -152,6 +152,7 @@ func (b S3Backend) PutMetadata(key string, m backends.Metadata) (err error) {
 		Key:               aws.String(key),
 		CopySource:        aws.String("/" + b.bucket + "/" + key),
 		Metadata:          mapMetadata(m),
+		MetadataDirective: aws.String("REPLACE"),
 	})
 	if err != nil {
 		return