# File lib/aws/core/signature/version_4.rb, line 27
        def add_authorization! credentials
          datetime = Time.now.utc.strftime("%Y%m%dT%H%M%SZ")
          headers['content-type'] ||= 'application/x-www-form-urlencoded'
          headers['host'] = host
          headers['x-amz-date'] = datetime
          headers['x-amz-security-token'] = credentials.session_token if
            credentials.session_token
          headers['x-amz-content-sha256'] ||= hexdigest(body || '')
          headers['authorization'] = authorization(credentials, datetime)
        end