Fix callable function class

[audiobird]

Fixes callable function class which wouldn't compile for me. Is there a chance I was using it wrong?

auto capture{0};
auto func = Function<int(int, int)>{[&capture](int a, int b) {
	capture += 10;
	return a + b + capture;
}};
auto out = func(10, 10); // no matching function call error

Also adds unit test

[danngreen]

Not sure how that error got in there, good find!