fix: Headers inspect for duplicate Set-Cookie values (#35461)
## Summary
- fix `Headers` custom inspection so duplicate iterable header names are
folded instead of overwritten
- add formatting-tolerant regression coverage for duplicate `Set-Cookie`
values with `Expires` dates
- add Node compatibility coverage for `ServerResponse.setHeaders(new
Headers())` preserving multiple Set-Cookie headers
Closes #35451.
## Tests
- `git diff --check`
- `cargo test -p unit_tests --test unit --
customInspectReturnsCorrectHeadersFormat`
- `cargo test -p unit_node_tests --test unit_node --
"ServerResponse.setHeaders preserves Headers set-cookie entries"`
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Co-authored-by: Nathan Whitaker <nathan@deno.com>
