Use RFC1123Pattern format

[kzrnm]

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Description

CookieHandlingTests.MissingCookie_Issue768 depend on culture "en-US" because DateTime.ToString is culture aware.

DateTime.ToString("R") creates RFC1123 format string regardless of culture.

https://docs.microsoft.com/en-us/dotnet/standard/base-types/standard-date-and-time-format-strings?redirectedfrom=MSDN#RFC1123

CultureInfo.CurrentCulture = CultureInfo.GetCultureInfo("ja");

// 月, 06 12月 2021 12:37:43 GMT
Console.WriteLine($"{DateTime.UtcNow.AddHours(1).ToString("ddd, dd MMM yyyy HH:mm:ss")} GMT");

// Mon, 06 Dec 2021 12:37:43 GMT
Console.WriteLine($"{DateTime.UtcNow.AddHours(1):R}");

[FlorianRappl]

LGTM - thanks!