Relativistic Doppler shifts are now applied to ``SpectralCoord`` in the correct
direction even if the ``astropy.units.spectral()`` equivalency is enabled.
Previously enabling the equivalency could cause wrong results with the
``SpectralCoord.to_rest()``,
``SpectralCoord.with_observer_stationary_relative_to()`` or
``SpectralCoord.with_radial_velocity_shift()`` methods.
