Remove dependency to System.Web.HttpContext

[di97mni]

System.Web.HttpContext.Current is null when running self host and you will get NullReferenceException below

https://github.com/WebApiContrib/WebApiContrib.Formatting.Xlsx/blob/master/src/WebApiContrib.Formatting.Xlsx/XlsxMediaTypeFormatter.cs#L90

[jordangray]

Eugh, I hadn't considered that; bet it's annoyed a fair few people already. I'm not really conveniently around my computer this week, but I'll throw a null check in there some time in the next fortnight.

Thanks for letting me know!