nditer multi_index gives incorrect value for 0-dim arrays

(From discussion in #445)

```
In [32]: a
Out[32]: array(0)

In [33]: a.shape
Out[33]: ()

In [34]: i = np.nditer(a, flags=["multi_index"])

In [35]: i.next()
Out[35]: array(0)

In [36]: i.multi_index
Out[36]: (0,)

In [37]: a[i.multi_index]
IndexError: 0-d arrays can only use a single () or a list of newaxes (and a single ...) as an index
```

The multi_index here should be `()`, not `(0,)`.
