Better support for `ActionResult<T>`

[cremor]

Continuation of #21

Instead of having to write

ActionResult<TestDto> result = await _controller.GetTestItemAsync(id);
result.Result.Should().BeNotFoundResult();

I'd like to write

ActionResult<TestDto> result = await _controller.GetTestItemAsync(id);
result.Should().BeNotFoundResult();

[faddiv]

I never got the admin privilege on the nuget package. Also, I didn't use this for a long time. I'm afraid this project died.