# File lib/aws/s3/request.rb, line 173
      def add_authorization!(signer)
        if signer.respond_to?(:session_token) and
            token = signer.session_token
          headers["x-amz-security-token"] = token
        end
        signature = URI.escape(signer.sign(string_to_sign, 'sha1'))
        headers["authorization"] = "AWS #{signer.access_key_id}:#{signature}"
      end