Preserve column order in YAML export (#287)

[vineethsaivs]

Fixes #287.

yaml.safe_dump sorts mapping keys alphabetically by default, so YAML export reordered columns inconsistently with the CSV and JSON exports (which keep the dataset's header order):

import tablib
d = tablib.Dataset()
d.headers = ["name", "id", "zebra", "apple"]
d.append(["x", 1, "z", "a"])

d.csv.splitlines()[0]   # 'name,id,zebra,apple'
d.yaml                  # before: '- {apple: a, id: 1, name: x, zebra: z}'
                        # after:  '- {name: x, id: 1, zebra: z, apple: a}'

This passes sort_keys=False when dumping the dataset and the databook, so YAML keeps the original column order (matching CSV/JSON).

test_yaml_export is updated to expect header order, and a focused regression test (test_yaml_export_preserves_column_order) is added; it fails on master (alphabetical) and passes with this change.

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Disclosure: prepared with AI assistance; reviewed by me and I can explain every line.

[claudep]

I think you could remove the "Regression test for #287" part, the rest of the patch looks totally good. Thanks!

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.16%. Comparing base (7503651) to head (811feda).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #644      +/-   ##
==========================================
+ Coverage   93.14%   93.16%   +0.01%     
==========================================
  Files          29       29              
  Lines        3226     3231       +5     
==========================================
+ Hits         3005     3010       +5     
  Misses        221      221              

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[vineethsaivs]

Thanks for the review! I've removed the "Regression test for #287" wording from the docstring; the test now reads as a plain description of the expected behavior.

[claudep]

Thanks!