BUG: _ones_like not in quantity_helper, hence  q**0 fails

Currently,
```
import astropy.units as u
u.Quantity(np.arange(10.), u.Jy)**0
```
yields
```
ERROR: TypeError: Unknown ufunc _ones_like.  Please raise issue on https://github.com/astropy/astropy [astropy.units.quantity]
```
This PR adds the "private" ufunc `np.core.umath._ones_like` to `quantity_helper.py` (and adds a test case).