Silent bug when indexing with boolean scalars

I think this is an edge case that should fail, it does if we use a 0d xarray as indexer, but not if using a numpy array or a python bool:

```python
import xarray as xr

x = xr.DataArray(np.arange(9).reshape((3, 3)), dims=("a", "b"))
x[np.array(True)]
# <xarray.DataArray (b: 3)> Size: 24B
# array([3, 4, 5])
# Dimensions without coordinates: b
```
Seems to be treating it simply as an integer