Issue with PATCH and CORS

CorsMessageHandler.cs

public class CorsMessageHandler : DelegatingHandler
    {
        const string Origin = "Origin";
        const string AccessControlRequestMethod = "Access-Control-Request-Method";
        const string AccessControlRequestHeaders = "Access-Control-Request-Headers";
        const string AccessControlAllowOrigin = "Access-Control-Allow-Origin";
        const string AccessControlAllowMethods = "Access-Control-Allow-Methods";
        const string AccessControlAllowHeaders = "Access-Control-Allow-Headers";
        const string AccessControlAllowCredentials = "Access-Control-Allow-Credentials";

        protected override Task<HttpResponseMessage> SendAsync(HttpRequestMessage request, CancellationToken cancellationToken)
        {

            IEnumerable<string> originHeaders;
            if (request.Headers.TryGetValues(Origin, out originHeaders))
            {
                return ProcessCorsRequest(request, ref cancellationToken, originHeaders.First());
            }
            return base.SendAsync(request, cancellationToken);
        }

        private Task<HttpResponseMessage> ProcessCorsRequest(HttpRequestMessage request, ref CancellationToken cancellationToken, string originHeader)
        {
            if (request.Method == HttpMethod.Options)
            {
                return Task.Factory.StartNew(() =>
                {
                    HttpResponseMessage response = new HttpResponseMessage(HttpStatusCode.OK);
                    AddCorsResponseHeaders(request, response, originHeader);
                    return response;
                }, cancellationToken);
            }
            else
            {
                return base.SendAsync(request, cancellationToken).ContinueWith<HttpResponseMessage>(task =>
                {
                    HttpResponseMessage resp = task.Result;
                    resp.Headers.Add(AccessControlAllowOrigin, originHeader);
                    resp.Headers.Add(AccessControlAllowCredentials, "true");
                    return resp;
                }, CancellationToken.None, TaskContinuationOptions.OnlyOnRanToCompletion, TaskScheduler.Current);
            }
        }

        private static void AddCorsResponseHeaders(HttpRequestMessage request, HttpResponseMessage response, string originHeader)
        {
            response.Headers.Add(AccessControlAllowOrigin, originHeader);
            response.Headers.Add(AccessControlAllowCredentials, "true");

            IEnumerable<string> accessControlRequestMethod;
            if (request.Headers.TryGetValues(AccessControlRequestMethod, out accessControlRequestMethod))
            {
                response.Headers.Add(AccessControlAllowMethods, accessControlRequestMethod.First());
            }

            IEnumerable<string> accessControlRequestHeaders;
            if (request.Headers.TryGetValues(AccessControlRequestHeaders, out accessControlRequestHeaders))
            {
                string requestedHeaders = string.Join(", ", accessControlRequestHeaders);
                response.Headers.Add(AccessControlAllowHeaders, requestedHeaders);
            }

        }
    }

HandlerConfig.cs

public class HandlerConfig
    {
        public static void RegisterHandlers(Collection<DelegatingHandler> handlers)
        {
            handlers.Add(new CorsMessageHandler());
           //...
        }
  }