# File lib/aws/core/signature/version_3.rb, line 26
        def add_authorization! credentials

          headers["x-amz-date"] ||= (headers["date"] ||= Time.now.httpdate)
          headers["host"] ||= host

          headers["x-amz-security-token"] = credentials.session_token if
            credentials.session_token

          # compute the authorization
          headers["x-amzn-authorization"] =
            "AWS3 "+
            "AWSAccessKeyId=#{credentials.access_key_id},"+
            "Algorithm=HmacSHA256,"+
            "SignedHeaders=#{headers_to_sign.join(';')},"+
            "Signature=#{signature(credentials)}"
        end