Fixing missing coordinate unit in aperture_photometry return for single ...

...positions

Sadly ``hub`` is not working out of the box, thus I couldn't attached this directly to #212. 
Part of the problem causing this bug was that the inner container of ``SkyCoord`` is not as consistent as ``Quantity``'s for single and multiple elements. I'm not sure whether this is a feature or just accidental, if the latter I'll open an issue for it in core. 

```
In [325]: coord_columns_m
Out[324]: 
(<Quantity [ 99., 99.] pix>,
 <Quantity [ 49., 49.] pix>,
 <SkyCoord (ICRS): (ra, dec) in deg
     [(0.0, 0.0), (0.0, 0.0)]>)  

In [325]: coord_columns_s
Out[325]: 
(<Quantity [ 99.] pix>,
 <Quantity [ 49.] pix>,
 <SkyCoord (ICRS): ra=0.0 deg, dec=0.0 deg>)
```

fixes #212