React to HttpAbstractions namespace changes

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

Author: dougbu

test/Microsoft.AspNetCore.StaticFiles.Tests/StaticFileContextTest.cs

@@ -6,7 +6,6 @@
 using System.IO;
 using Microsoft.AspNetCore.Builder;
 using Microsoft.AspNetCore.Http;
-using Microsoft.AspNetCore.Http.Internal;
 using Microsoft.Extensions.FileProviders;
 using Microsoft.Extensions.Logging.Testing;
 using Microsoft.Extensions.Primitives;