Fix WCSExt to properly parse FITS keyword types using parse_header_val

[cgarling]

Fixes #239. @andrew-saydjari could you demo this with your workflow to verify it fixes the problem you noticed?

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.09%. Comparing base (6df4964) to head (f6bc600).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #240      +/-   ##
==========================================
- Coverage   92.10%   92.09%   -0.02%     
==========================================
  Files           7        7              
  Lines         722      721       -1     
==========================================
- Hits          665      664       -1     
  Misses         57       57              

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