array[slice_group_from_an_incompatible_axis] is buggy

I am pretty sure there are cases where the group is retargeted to the correct axis, but at least in this case it isn't while it should:

```python
>>> arr = ndtest(3)
>>> a2 = Axis('a=a0,a1')
>>> arr[a2]  # <-- correct
a  a0  a1
    0   1
>>> arr[a2[:]]  # <-- wrong
a  a0  a1  a2
    0   1   2
```