Bug in wcs1d-fits reader and flux_unit

I'm pretty sure there's a bug in the wcs1d-fits reader. I have some spectra that have `BUNIT = erg/A/s/cm2`. This is not able to be parsed into `erg / (Angstrom s cm2)`, which I am "ok" with but the bigger issue is that if I give `flux_unit = u.Unit("erg / (angstrom s cm2)")` , I get an "unable to convert" error. The expected behavior is for flux_unit to IGNORE BUNIT, not try to convert to it. I'll upload an example file later.

Start with the wcs1d-fits reader and reproduce the issue using BUNIT="erg/A/s/cm2" together with flux_unit=u.Unit("erg / (angstrom s cm2)"). Trace how flux_unit and BUNIT are handled; done means an explicit flux_unit bypasses BUNIT conversion without the unable-to-convert error, with regression coverage for this case.

Written by the indexing model from the issue text.