def signature
      @xml or to_xml

      digest  = OpenSSL::Digest::Digest.new('sha1')
      OpenSSL::HMAC.digest(digest, @merchant_key, @xml)
    end
def signature
      @xml or to_xml

      digest  = OpenSSL::Digest::Digest.new('sha1')
      OpenSSL::HMAC.digest(digest, @merchant_key, @xml)
    end
Returns the signature for the cart XML.