fix read_spectra skipping exp_fibermap

[sbailey]

This PR is a followup to #2052, fixing a bug with read_spectra(..., skip_hdus=['EXP_FIBERMAP', ...])

The original code looping over HDUs had

...
elif name == "EXP_FIBERMAP" and name not in skip_hdus:
    ...
elif ...

The problem was that if "EXP_FIBERMAP" was in skip_hdus, then it would continue down the elif chain and eventually get interpreted as a catchall user-provided "BAND_EXTRANAME" image HDU, which would then trip on reading a table as an image.

The fix is

...
elif name == "EXP_FIBERMAP":
    if name not in skip_hdus:
        ...
elif ...

so that if it finds "EXP_FIBERMAP" but it is in skip_hdus then it will stop checking rather than continuing to try to parse the name as meaning other things. I applied that same pattern to all other cases of known HDU names that might be in the skip_hdu list.

This includes a unit test that would have caught the original problem, but now passes.

@dirkscholte I'm pretty sure that I introduced this bug when updating your PR #2052, but if you could double-check my fix here I would appreciate it. Thanks.

[dirkscholte]

Hi @sbailey. This makes a lot of sense! In the earliest version I made of this (might not be the version that started this pull request) I purposefully broke out some functionality because I had a similar issue. Thanks for fixing this :)