WCS should set default values for RADESYS

@mdboom - the FITS-WCS standard says

RADESYSa – [character; default: FK4, FK5, or ICRS: see be- low]. Name of the reference frame of equatorial or ecliptic coordinates, whose value must be one of those specified in Table 24. The default value is FK4 if the value of EQUINOXa < 1984.0, FK5 if EQUINOXa ≥ 1984.0, or ICRS if EQUINOXa is not given.

Currently ``wcs.wcs.radesys`` defaults to ``''``. Can we make it default to the above values instead? I'd be happy to try and do this, but is there a specific place this should go?