title: How do you stub User.Identity.GetUserId() in ASP MVC 5 (Microsoft.AspNet.Identity)
body: <p>How do you stub User.Identity.GetUserId() in ASP MVC 5 (Microsoft.AspNet.Identity) for a unit test of a MVC5 Controller? GetUserId() is an extension, so I can't mock it directly. And I need to assign the Id before the test.
  It seems that you have to create a Claim and assign it to a GenericIdentity. But it seems like a lot to do for a unit test. Do you know of any alternatives? </p>