Inconsistant expectation for location of header data (vs. other metadata)

The location of header keywords is inconsistent - wcs1d_fits writer will update the output header with anything in the spectrum.meta dictionary. Tabular fits expects header-specific keys to be in meta.header, as a fits.Header object (and will ignore it if its a dictionary). I'm not sure what the other readers/writers do, but it is inconsistent at least in these. Because there can be other information in `meta` that might not belong in the fits file header, meta.header is preferable (either as dict or `Header`?)

(Issue made after discussion with @kelle @havok2063, @tepickering)

Start by comparing the wcs1d_fits writer with Tabular fits handling of spectrum.meta and meta.header, then inspect what the other readers and writers do. Resolve whether header metadata should consistently use meta.header as a dict or fits.Header, and define completion as consistent behavior across the affected readers and writers.

Written by the indexing model from the issue text.