Calling `Array` on a zero dimensional variable returns a scalar

```julia
using NCDatasets
fn = tempname() * ".nc"
ds = NCDatasets.Dataset(fn, "c")
NCDatasets.defVar(ds, "a", Int32, ())
```

Then `Array` returns a scalar:
```juia
julia> Array(ds["a"])
-2147483647
```

It should always return an `Array` (even if its zero-dimensional)