React to HttpAbstractions namespace changes

- aspnet/HttpAbstractions#549 and aspnet/HttpAbstractions#592
- clean up `using`s

Author: dougbu

samples/Mvc.RenderViewToString/RazorViewToStringRenderer.cs

@@ -3,7 +3,7 @@
 
 using System;
 using System.IO;
-using Microsoft.AspNetCore.Http.Internal;
+using Microsoft.AspNetCore.Http;
 using Microsoft.AspNetCore.Mvc;
 using Microsoft.AspNetCore.Mvc.Abstractions;
 using Microsoft.AspNetCore.Mvc.ModelBinding;