'SpectralGWCS' object has no attribute 'to_fits'

Loading an SDSS spectrum with the 'SDSS-I/II spSpec' format loader and then writing to fits using format "wcs1d-fits" writer raises error:

ValueError: Only Spectrum1D objects with valid WCS can be written as wcs1d: 'SpectralGWCS' object has no attribute 'to_fits'

Please see attached fits file and minimum example script.
We include in the script the output of running the script and environment info.
[SpectralGWCSBug.tar.gz](https://github.com/astropy/specutils/files/5594130/SpectralGWCSBug.tar.gz)

Start with the attached SpectralGWCSBug.tar.gz, including its minimum example, output, and environment information. Reproduce loading the SDSS-I/II spSpec file and writing it with the wcs1d-fits format, then trace why SpectralGWCS lacks to_fits where the writer validates WCS support. Done means the example writes successfully without the reported ValueError.

Written by the indexing model from the issue text.