Better support for `ActionResult<T>`

Continuation of #21

Instead of having to write
```cs
ActionResult<TestDto> result = await _controller.GetTestItemAsync(id);
result.Result.Should().BeNotFoundResult();
```

I'd like to write
```cs
ActionResult<TestDto> result = await _controller.GetTestItemAsync(id);
result.Should().BeNotFoundResult();
```