cumsum error

I'm just trying to make a function for the arclength along a space-curve.

```julia
curve(u) = [1.,0.,cos(u)]
dist = arclength(curve)
speed = Fun(u->hypot(derivative(curve,u)...),0..pi)
sum(speed) # 2.0 works!
cumsum(speed)(pi) # 0.0 doesn't work!
```