C# - Get Absolute URL From Relative Path (refactored Method)

I am really surprised that there is no native .NET method to get an absolute url from a relative url. I know this has been discussed many times, but never have come across a satisfactory method that handles this well. I think all I need left is to auto choose the protocol instead of hard coding it (http/https). Anything else I am missing (caveats, performance, etc)?

public static string GetAbsoluteUrl(string url)
{
//VALIDATE INPUT FOR ALREADY ABSOLUTE URL
if (url.StartsWith("http://", StringComparison.OrdinalIgnoreCase) || url.StartsWith("https://", StringComparison.OrdinalIgnoreCase))
{
return url;
}
//GET PAGE REFERENCE FOR CONTEXT PROCESSING
Page page = HttpContext.Current.Handler as Page;
//RESOLVE PATH FOR APPLICATION BEFORE PROCESSING
if (url.StartsWith("~/"))
{
url = page.ResolveUrl(url);
}
//BUILD AND RETURN ABSOLUTE URL
return "http://" + page.Request.ServerVariables["SERVER_NAME"] + "/" + url.TrimStart('/');
}