React to HttpAbstractions namespace changes

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

Author: dougbu

test/Microsoft.AspNetCore.Diagnostics.Tests/ElmMiddlewareTest.cs

@@ -6,12 +6,9 @@
 using System.Security.Claims;
 using System.Text;
 using System.Threading.Tasks;
-using Microsoft.AspNetCore.Builder;
 using Microsoft.AspNetCore.Diagnostics.Elm;
 using Microsoft.AspNetCore.Http;
 using Microsoft.AspNetCore.Http.Features;
-using Microsoft.AspNetCore.Http.Features.Internal;
-using Microsoft.AspNetCore.Http.Internal;
 using Microsoft.Extensions.Logging;
 using Microsoft.Extensions.Options;
 using Moq;