问Request.GetOwinContext在单元测试中返回null -如何在单元测试中测试OWIN身份验证？

EN

[TestMethod]
public void TestRegister()
{
    using (WebApp.Start<Startup>("localhost/myAPI"))
    using (AccountController ac = new AccountController()
        {
            Request = new System.Net.Http.HttpRequestMessage
                (HttpMethod.Post, "http://localhost/myAPI/api/Account/Register")
        })
    {
        var result = ac.Register(new Models.RegisterBindingModel()
        {
            Email = "testemail@testemail.com",
            Password = "Pass@word1",
            ConfirmPassword = "Pass@word1"
        }).Result;
        Assert.IsNotNull(result);
    }
}

Result Message: 
Test method myAPI.Tests.Controllers.AccountControllerTest.TestRegister 
    threw exception: 
System.ArgumentNullException: Value cannot be null.
Parameter name: context
Result StackTrace:  
at Microsoft.AspNet.Identity.Owin.OwinContextExtensions
    .GetUserManager[TManager](IOwinContext context)
at myAPI.Controllers.AccountController.get_UserManager()
...

public void ConfigureAuth(IAppBuilder app)
{
    HttpConfiguration config = new HttpConfiguration();
    config.MapHttpAttributeRoutes();
    app.UseWebApi(config);

    // Configure the db context and user manager to use a single 
    //  instance per request
    app.CreatePerOwinContext(ApplicationDbContext.Create);
    app.CreatePerOwinContext<ApplicationUserManager>
        (ApplicationUserManager.Create);

    // Enable the application to use a cookie to store information for 
    //  the signed in user
    //  and to use a cookie to temporarily store information about a 
    //  user logging in with a third party login provider
    app.UseCookieAuthentication(new CookieAuthenticationOptions());
    app.UseExternalSignInCookie(DefaultAuthenticationTypes.ExternalCookie);

    // Configure the application for OAuth based flow
    PublicClientId = "self";
    OAuthOptions = new OAuthAuthorizationServerOptions
    {
        TokenEndpointPath = new PathString("/Token"),
        Provider = new ApplicationOAuthProvider(PublicClientId),
        AuthorizeEndpointPath = new PathString("/api/Account/ExternalLogin"),
        AccessTokenExpireTimeSpan = TimeSpan.FromDays(14),
        AllowInsecureHttp = true
    };

    // Enable the application to use bearer tokens to authenticate users
    app.UseOAuthBearerTokens(OAuthOptions);
}

// POST api/Account/Register
[AllowAnonymous]
[Route("Register")]
public async Task<IHttpActionResult> Register(RegisterBindingModel model)
{
    if (!ModelState.IsValid)
    {
        return BadRequest(ModelState);
    }

    var user = new ApplicationUser() 
       { UserName = model.Email, Email = model.Email };

    IdentityResult result = await UserManager.CreateAsync(user, model.Password);

    if (!result.Succeeded)
    {
        return GetErrorResult(result);
    }

    return Ok();
}

public ApplicationUserManager UserManager
{
    get
    {
        return _userManager ?? Request.GetOwinContext()
           .GetUserManager<ApplicationUserManager>();
    }
    private set
    {
        _userManager = value;
    }
}

return _userManager ?? Request.GetOwinContext()
    .GetUserManager<ApplicationUserManager>();