Preserve headers when copying an empty table

[OskarEichler]

Summary

Pass the current headers to the new table in by_col, by_row, and by_col_or_row. Preserve the existing shallow row-copy behavior.

Reproduction

CSV.parse("a,b\n", headers: true).by_col.headers returns [] instead of ["a", "b"]; appending data then assigns nil headers. The same problem affects the other two non-mutating access-mode methods.

Verification

48 focused checks: 18 failures on the release, zero with the patch, covering empty tables, duplicate/nil/symbol headers, appends, and original-table isolation.

This isolated patch passes the unchanged upstream suite in both normal and CSV_PARSER_SCANNER_TEST=yes modes: 527 tests / 4,050 assertions in each mode, zero failures/errors. Commands use Ruby 4.0.6 through rbenv (bundle exec ruby run-test.rb). Syntax and git diff --check pass. The combined release-based consumer branch also passes gem build and rake warning:error rdoc.

No tests were added or changed, per the requesting project's explicit no-new-tests policy. Focused checks and measurements live outside this repository. The upstream no-RuboCop policy is respected. Verification was local on macOS/Ruby 4.0.6; the supported OS/Ruby matrix was not run locally. Current main and installed 3.3.6 share base commit 0873ab362d996f12796c3c3e8998b5be657a9b12; no unrelated changes are included.

Breaking changes and limitations

None intended. Empty table copies now retain their headers. Row ownership semantics are unchanged.

[kou]

No tests were added or changed, per the requesting project's explicit no-new-tests policy.

Where is it written?

[kou]

Do you have any real-world problem case by this?