Add support for PATH_INFO server variable

Hi there,

Can you please provide support for the PATH_INFO server variable on IIS. I'm trying to migrate my legacy MVC website to the new one I've built ASP.Net Core. I'm trying to migrate the URL rewrite rules but they don't work as they rely on the PATH_INFO server variable.

I get the following error when trying to run my application:
Unrecognized parameter type: 'PATH_INFO', terminated at string index: '10'
Microsoft.AspNetCore.Rewrite.Internal.IISUrlRewrite.ServerVariables.FindServerVariable(string serverVariable, ParserContext context, UriMatchPart uriMatchPart)

Extract from my configure method in startup.cs which reads in an xml file:

```
using (StreamReader iisUrlRewriteStreamReader =
File.OpenText("IISUrlRewrite.xml"))
{
var options = new RewriteOptions()
.AddIISUrlRewrite(iisUrlRewriteStreamReader);

app.UseRewriter(options);
}
```

The example content of the IISUrlRewrite,xml file is:
```

```

If I change {PATH_INFO} to {QUERY_STRING}, the application loads but this isn't the redirect I need. Can you please provide support for this ASAP please?

In the meantime, is there a workaround I can use that will support {PATH_INFO} patterns on IIS?

Thanks