Fix FormData clone boundary inconsistency issue

dio/CHANGELOG.md

@@ -7,6 +7,7 @@ See the [Migration Guide][] for the complete breaking changes list.**
 
 - Update comments and strings with `MultipartFile`.
 - Removes redundant warnings when composing request options on Web.
+- Fixes boundary inconsistency in `FormData.clone()`.
 
 ## 5.7.0
 

dio/lib/src/form_data.dart

@@ -74,7 +74,7 @@ class FormData {
   ///
   /// See also: https://www.w3.org/Protocols/rfc1341/7_2_Multipart.html
   String get boundary => _boundary;
-  late final String _boundary;
+  late String _boundary;
 
   /// The form fields to send for this request.
   final fields = <MapEntry<String, String>>[];
@@ -210,6 +210,7 @@ class FormData {
   // Convenience method to clone finalized FormData when retrying requests.
   FormData clone() {
     final clone = FormData();
+    clone._boundary = _boundary;
     clone.fields.addAll(fields);
     for (final file in files) {
       clone.files.add(MapEntry(file.key, file.value.clone()));

dio/test/formdata_test.dart

@@ -176,6 +176,7 @@ void main() async {
         expect(fm1 != fm, true);
         expect(fm1.files[0].value.filename, fm.files[0].value.filename);
         expect(fm1.fields, fm.fields);
+        expect(fm1.boundary, fm.boundary);
       },
       testOn: 'vm',
     );