Include UnitSphericalTransform in represent_as shortcuts

This has just been *killing* me:  [master]
```
>>> from astropy.coordinates import SkyCoord
>>> sc = SkyCoord(1, 2, unit='deg')
>>> sc.ra
<Longitude 0.9999999999999887 deg>
>>> sc.dec
<Latitude 2.0 deg>
```
It also causes all kinds of problems writing documentation.  This PR fixes it.  [HEAD]:
```
>>> sc = SkyCoord(1, 2, unit='deg')
>>> sc.ra
<Longitude 1.0 deg>
>>> sc.dec
<Latitude 2.0 deg>
```
It was a simple matter of shortcutting the `represent_as` path to and from cartesian.  I didn't write any new tests as I assume the representation module has near 100% coverage.  :smile:

cc: @astrofrog @cadair @eteq