# File lib/aws/core/signers/version_3_https.rb, line 34
        def sign_request req
          parts = []
          parts << "AWS3-HTTPS AWSAccessKeyId=#{credentials.access_key_id}"
          parts << "Algorithm=HmacSHA256"
          parts << "Signature=#{signature(req)}"
          req.headers['x-amzn-authorization'] = parts.join(',')
          req.headers['x-amz-security-token'] = credentials.session_token if
            credentials.session_token
          req
        end