Using ServiceStack.Test JsonSerializer in a Web API MediaTypeFormatter

ServiceStackFormatter.cs

using System.Net.Http.Formatting;
using System.Net.Http.Headers;
using System.Text;
using System.Threading.Tasks;
using ServiceStack.Text;

namespace HttpClientTest.Formatters {
    public class ServiceStackFormatter : MediaTypeFormatter {
        public ServiceStackFormatter() {
            // Fill out the mediatype and encoding we support
            SupportedMediaTypes.Add(new MediaTypeHeaderValue("application/json"));
            Encoding = new UTF8Encoding(false, true);
        }

        protected override bool CanReadType(System.Type type) {
            return type != typeof(IKeyValueModel);
        }

        protected override bool CanWriteType(System.Type type) {
            return true;
        }

        protected override Task<object> OnReadFromStreamAsync(System.Type type, System.IO.Stream stream, HttpContentHeaders contentHeaders, FormatterContext formatterContext) {

            return Task.Factory.StartNew(() => JsonSerializer.DeserializeFromStream(type, stream));

        }

        protected override Task OnWriteToStreamAsync(System.Type type, object value, System.IO.Stream stream, HttpContentHeaders contentHeaders, FormatterContext formatterContext, System.Net.TransportContext transportContext) {

            return Task.Factory.StartNew(() => JsonSerializer.SerializeToStream(value, type, stream));

        }
    }
}