Keep SkyCoord differentials the same in represent_as if possible - #11482

from astropy.coordinates import SkyCoord
from astropy import units as u

sc = SkyCoord(10*u.deg, 10*u.deg, radial_velocity=-10*u.km/u.s)
sc
<SkyCoord (ICRS): (ra, dec) in deg
    (10., 10.)
 (radial_velocity) in km / s
    (-10.,)>

sc.data.differentials
{'s': <RadialDifferential (d_distance) in km / s
     (-10.,)>}

sc.represent_as('spherical')
<SphericalRepresentation (lon, lat, distance) in (rad, rad, )
    (0.17453293, 0.17453293, 1.)
 (has differentials w.r.t.: 's')>

sc.represent_as('spherical').differentials
{'s': <SphericalDifferential (d_lon, d_lat, d_distance) in (km rad / s, km rad / s, km / s)
     (2.22044605e-16, -2.22044605e-16, -10.)>}

sc.represent_as('spherical')
<SphericalRepresentation (lon, lat, distance) in (rad, rad, )
    (0.17453293, 0.17453293, 1.)
 (has differentials w.r.t.: 's')>

c.represent_as('spherical').differentials
Out[3]: 
{'s': <RadialDifferential (d_distance) in km / s
     (-10.,)>}