🐛 Fix FormData.clone() dropping boundaryName and camelCaseContentDisposition

[ultramcu]

Follow-on to merged #2305 and merged #2008. FormData.clone() still drops two of the original constructor options, so a retried multipart request silently re-serialises with the defaults.

1. Bug

clone() builds the new instance via the no-arg FormData() constructor:

FormData clone() {
  final clone = FormData();           // ← resets options to defaults
  clone._boundary = _boundary;        // (added by #2305)
  ...
}

That resets two options that the original may have set:

Option	Added in	Symptom on a cloned (retried) request
boundaryName	original API	clone.boundaryName returns the dio default, not the user's label
camelCaseContentDisposition	#2008	wire bytes contain content-disposition: form-data; … instead of Content-Disposition: …

The header-casing regression is the user-visible one: any server that matches Content-Disposition case-sensitively (some signing gateways, some upload validators) accepts the first attempt and rejects the retry.

2. Fix

Forward both options through clone():

FormData clone() {
  final clone = FormData(
    boundaryName: boundaryName,
    camelCaseContentDisposition: camelCaseContentDisposition,
  );
  clone._boundary = _boundary;
  clone.fields.addAll(fields);
  for (final file in files) {
    clone.files.add(MapEntry(file.key, file.value.clone()));
  }
  return clone;
}

One-line behavioural change, no new public API.

3. Test

New regression test in dio/test/formdata_test.dart (clone() preserves boundaryName and camelCaseContentDisposition):

• builds a FormData with boundaryName: '--custom-boundary' and camelCaseContentDisposition: true,
• clones it,
• asserts both getters round-trip,
• decodes readAsBytes() on the clone and asserts the body contains Content-Disposition: form-data; name="name" and does not contain the lower-case content-disposition: form.

Fail-before / pass-after verified locally:

• before the fix: the new test fails on the wire-body assertions (clone emits content-disposition: lower-case).
• after the fix: full dio/test/formdata_test.dart suite green (8/8).

4. CI

• dart format --output=none --set-exit-if-changed . on dio/ — clean (0 of 63 files changed).
• dart analyze on dio/lib/src/form_data.dart and dio/test/formdata_test.dart — no issues.
• Full dart test on the dio/ package — 220 passed / 6 network-gated skipped / 0 failed.

New Pull Request Checklist

• I have read the Documentation
• I have searched for a similar pull request in the project and found none
• I have updated this branch with the latest main branch to avoid conflicts (via merge from master or rebase)
• I have added the required tests to prove the fix/feature I'm adding
• I have updated the documentation (if necessary)
• I have run the tests without failures
• I have updated the CHANGELOG.md in the corresponding package

[github-actions]

Code Coverage Report: Only Changed Files listed

Package	Base Coverage	New Coverage	Difference
Overall Coverage	🟢 88.88%	🟢 88.89%	🟢 0.01%

Minimum allowed coverage is 0%, this run produced 88.89%