anchor.diff:  
       let ctx = Substitute.For<HttpContext>()
       mockXml ctx
  +    mockJson ctx (Newtonsoft None)
       mockNegotiation ctx config.NegotiationConfig
       let app =
           GET >=> choose [
  
positive.code: module Giraffe.Tests.HttpHandlerTests
  
  open System
  open System.IO
  open System.Collections.Generic
  open Microsoft.AspNetCore.Http
  
negative.code:         | None -> assertFailf "Result was expected to be %s" expected
          | Some ctx ->
              Assert.Equal(config.StatusCode, ctx.Response.StatusCode)
              if ctx.Response.StatusCode = 200 && config.ReturnContentType = "application/xml; charset=utf-8" then
                  let body = getBody ctx
                  XmlAssert.equals expected body
                  Assert.Equal(config.ReturnContentType, ctx.Response |> getContentType)
  