public HttpResponseMessage Register(model)
{
    return new HttpResponseMessage
            {
                StatusCode = HttpStatusCode.BadRequest,
                Content = new ObjectContent<List<string>>(errors, Configuration.Formatters.JsonFormatter)
            };
}

[TestMethod]
        public void Register_ReturnErrorsWithBadRequest_IfValidationFails()
        {
            PostUserRegistration model = new PostUserRegistration {
                Name = "Wai Yan Hein",
                Email = "waiyanhein@gmail.com",
                Password = ""
            };

            Mock<JsonMediaTypeFormatter> formatterMock = new Mock<JsonMediaTypeFormatter>();
            Mock<MediaTypeFormatterCollection> formatterCollection = new Mock<MediaTypeFormatterCollection>();
            formatterCollection.Setup(x => x.JsonFormatter).Returns(formatterMock.Object);
            Mock<HttpConfiguration> httpConfigMock = new Mock<HttpConfiguration>();
            httpConfigMock.Setup(x => x.Formatters).Returns(formatterCollection.Object);
            Mock<IAccountRepo> accRepoMock = new Mock<IAccountRepo>();

            AccountsController controller = new AccountsController(accRepoMock.Object);

            controller.Configuration = httpConfigMock.Object;
            controller.ModelState.AddModelError("", "Faking some model error");
            HttpResponseMessage response = controller.Register(model);
            Assert.AreEqual(response.StatusCode, System.Net.HttpStatusCode.BadRequest);
        }