Resolves #40.

If I understood correctly, this solves issue #40. 

You will now be able to write custom assertion methods like this one:

```c#
public static class ControllerResultTestExtensions
{
    public static void ShouldGiveHttpUnauthorizedResult<TController>(
        this ControllerResultTest<TController> result) where TController : Controller
    {
        result.ValidateActionReturnType<HttpUnauthorizedResult>();
        var httpUnauthorizedResult = (HttpUnauthorizedResult) result.ActionResult;
        if (httpUnauthorizedResult.StatusCode != 1337)
        {
        }
    }
}
```