code: public String constructUnsignedPayload() {
      // TODO reverse order when V4 becomes default
      if (Storage.SignUrlOption.SignatureVersion.V4.equals(signatureVersion)) {
        return constructV4UnsignedPayload();
      }
      return constructV2UnsignedPayload();
    }
docs: Constructs payload to be signed.
  
  @return payload to sign
  @see <a href="https://cloud.google.com/storage/docs/access-control#Signed-URLs">Signed URLs</a>