Fix for app relative auth redirects when absolute host uri inferring fails

The issue occurs when using an AuthService HtmlRedirect of e.g. "~/login.html". That redirect only works when the absolute uri can be resolved via settings like `EndpointHost.Config.WebHostUrl`.

Not sure if my fix is suitable though... e.g. how does using `VirtualPathUtility.ToAbsolute` affects self hosted services? Or if the modification I made should be pushed lower down and made part of the `IHttpRequest.GetBaseUrl()` implementation?