using var httpTest = new HttpTest();
httpTest
    .ForCallsTo("github.com")
    .RespondWith(HttpRequestMessage request => {
       // create a body based on the request
       var body = request.Headers["RequestIdentifier"]
       return new CapturedStringContent(body);
    })

    var resp = setup.GetNextResponse(request) ?? ...

    private readonly List<Func<HttpRequestMessage, HttpResponseMessage>> _responses = new List<Func<HttpRequestMessage, HttpResponseMessage>>();
...
    internal HttpResponseMessage GetNextResponse(HttpRequestMessage request) {
        if (!_responses.Any())
            return null;
            // atomically get the next response in the list, or the last one if we're past the end
            return _responses[Math.Min(Interlocked.Increment(ref _respIndex), _responses.Count) - 1](request);
    }

    // removal of null from buildContent
    public HttpTestSetup RespondWith(Func<HttpContent> buildContent, int status = 200, object headers = null, object cookies = null, bool replaceUnderscoreWithHyphen = true) {
        return RespondWith(request => buildContent?.Invoke(), status, headers, cookies, replaceUnderscoreWithHyphen);
    }

    // new method
    public HttpTestSetup RespondWith(Func<HttpRequestMessage, HttpContent> buildContent = null, int status = 200, object headers = null, object cookies = null, bool replaceUnderscoreWithHyphen = true) {
    return RespondWith(request => {
        var response = new HttpResponseMessage {
            StatusCode = (HttpStatusCode)status,
            Content = buildContent?.Invoke(request)
        };
    }
    
    // absolute base method
    public HttpTestSetup RespondWith(Func<HttpRequestMessage, HttpResponseMessage> buildResponse) {
        _responses.Add(buildResponse);
        return this;
    }