Use `UNSIGNED_PAYLOAD` to check signature if x-amz-content-sha256 isn't provided as signed header
https://docs.aws.amazon.com/AmazonS3/latest/API/sig-v4-header-based-auth.html
" You include the literal string UNSIGNED-PAYLOAD when constructing a canonical request, and set the same value as the x-amz-content-sha256 header value when sending the request to Amazon S3"
Signed-off-by: Denis Kirillov <d.kirillov@yadro.com>