Ensure equivalencies are copied over in conversions

While trying to address #1271, I noticed that equivalencies are lost during conversions, and hence the following fails:
```
import astropy.units as u; from astropy.units import Quantity
f = u.Quantity([1000.,2000.],u.Hz, equivalencies=u.spectral())
f.to(u.nm).to(u.Hz)
ERROR: UnitsException: 'nm' (length) and 'Hz' (frequency) are not convertible [astropy.units.core]
```
(just `f.to(u.nm)` works fine)
This one-line-change ensures that whatever equivalencies are used in `q.to(...)` are kept in the newly created quantity.