Hi,

I was going over the codechanges to see what we need to do to migrate from 2.x to 3.x and I think I spotted a bug.

This method, located in Microsoft.ApplicationInsights.Web.Implementation.HttpRequestExtensions, line 18

 public static string UnvalidatedGetHeader(this HttpRequest httpRequest, string headerName)
 {
    string value = httpRequest.Unvalidated.Headers[headerName];
    return string.Empty;
 }

Always returns and empty string, I'd expect it to return the value of the header (or a comment as to why an empty string is always returned).

I could create a PR if needed, but it might be faster to do for someone who has already contributed to this project. Let me know what you need.