Fix FormData clone boundary inconsistency issue

dio/lib/src/form_data.dart

@@ -19,12 +19,16 @@ String get _nextRandomId =>
 
 /// A class to create readable "multipart/form-data" streams.
 /// It can be used to submit forms and file uploads to http server.
+///
+/// Note: When initBoundary is specified, it will override the boundaryName configuration.
+/// This is because the actual boundary is composed of boundaryName plus a random string.
 class FormData {
   FormData({
+    String? initBoundary,
     this.boundaryName = _boundaryName,
     this.camelCaseContentDisposition = false,
   }) {
-    _init();
+    _init(initBoundary: initBoundary);
   }
 
   /// Create [FormData] from a [Map].
@@ -45,11 +49,16 @@ class FormData {
   final bool camelCaseContentDisposition;
 
   void _init({
+    String? initBoundary,
     Map<String, dynamic>? fromMap,
     ListFormat listFormat = ListFormat.multi,
   }) {
-    // Get an unique boundary for the instance.
-    _boundary = '$boundaryName-$_nextRandomId';
+    if(initBoundary != null) {
+      _boundary = initBoundary;
+    }else{
+      // Get an unique boundary for the instance.
+      _boundary = '$boundaryName-$_nextRandomId';
+    }
     if (fromMap != null) {
       // Use [encodeMap] to recursively add fields and files.
       // TODO(Alex): Write a proper/elegant implementation.
@@ -209,7 +218,7 @@ class FormData {
 
   // Convenience method to clone finalized FormData when retrying requests.
   FormData clone() {
-    final clone = FormData();
+    final clone = FormData(initBoundary: _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',
     );