Update get_walked_pages to support custom post types Closes #200

[zachgibb]

Description of the Change

When reordering a custom post type, this get_walked_pages call fails with an undefined array key error on line 433.

Digging into it more, we can see that the get_walked_pages call is outputting only the top_level_pages for the page post type.

This is due to the get_pages call on line 200 (https://github.com/10up/simple-page-ordering/blob/develop/class-simple-page-ordering.php#L200) defaulting to post_type => 'page'.

If we tweak this to pass in the post_type of the post we're working with, in the same way we do with is_post_type_sortable, this issue will be resolved.

Closes #200

How to test the Change

• Create a new post type
• Create a parent and child post within this new post type
• Verify debug warning does not get displayed

Changelog Entry

Fixed error in call to get_walked_pages for custom post types

Credits

Props @zachgibb

Checklist:

• [x] I agree to follow this project's Code of Conduct.
• [ ] I have updated the documentation accordingly.
• [ ] I have added tests to cover my change.
• [ ] All new and existing tests pass.