Wrong sns message signature

[DayS1eeper]

Expected behaviour (SNS)
In Verifying the Signatures of Amazon SNS Messages https://docs.aws.amazon.com/sns/latest/dg/sns-verify-signature-of-message.html is specified that string to sign must contain Subject name-value pair if it is included in the message. It doesn't mean that Subject must be omitted if value of Subject key is empty string. In aws-php-sns-message-validator Subject field is omitted only when message doesn't contain it https://github.com/aws/aws-php-sns-message-validator/blob/ba6810807ac8936317021ccc14cdb1297d525ae1/src/MessageValidator.php#L162 , the same logic is present in Example Code for an Amazon SNS Endpoint Java Servlet https://docs.aws.amazon.com/sns/latest/dg/sns-example-code-endpoint-java-servlet.html

Current behaviour (goaws)
Subject is omitted while creating string to sign if the value is empty string
https://github.com/p4tin/goaws/blob/6ecd9bf473ad2b2a88dba5224957826665710dec/app/gosns/gosns.go#L257

It would be great if it were fixed.

[p4tin]

As I am involved in other projects at the moment and you seem to have a grasp of the issue, I'd very much welcome a PR.