Gridded(Constant{Previous}()) returns wrong results

To be fair, the docs for [Constant ](https://docs.juliahub.com/General/Interpolations/stable/api/#Interpolations.Constant) does mention possibility of numerical errors but this seems quite wrong...

```
# wrong - should be 20
julia> interpolate(([1,2],),[10,20],Gridded(Constant{Previous}()))(2)
10

# correct
julia> interpolate([10,20],BSpline(Constant{Previous}()))(2)
20.0
```