Missing .ThrowsAsync() extension for Task-returning methods

[gzak]

Currently, there's only a ThrowsAsync extension for methods which return Task, but there's no equivalent for methods which return the non-generic Task type (a "void" Task, essentially).

The same is true for ReturnsAsync, though it's questionable how useful it would be.

[neraath]

+1

[stakx]

I've prepared an implementation for ThrowsAsync for non-generic Tasks, which I'll be PR-ing in the next few days, but I'm omitting ReturnsAsync for now. I can't see any practical use for it.