Do not require that function arguments are ::Function.

[tpapp]

Fixes #224.

[tpapp]

Style note: tell me if the struct I added for testing should be somewhere else, happy to move it. It was moved up front to make tests work on 1.0.

[wesselb]

Hmmmm, @oxinabox or @willtebbutt, could either of you confirm whether this change could have any unintended consequences? I can't really recall if letting f be a function Function had a particular reason or not.

[willtebbutt]

I can't see it being a problem.

[oxinabox]

If tests pass then all is well

[oxinabox]

thanks