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PR Title
Add a setter for HttpContext.Current

PR Description

Looking at the System.Web implementation, it seems like HttpContext.Current is not read-only https://github.com/microsoft/referencesource/blob/master/System.Web/HttpContext.cs#L566-L579 which is different than our implementation https://github.com/dotnet/systemweb-adapters/blob/main/src/Microsoft.AspNetCore.SystemWebAdapters/HttpContext.cs#L31

Addresses #400