Fix: Bug#126 flutter_ume_kit_dio_plus failed to parse request headers

修复问题：bytedance#126

(cherry picked from commit d8ad830)

kits/flutter_ume_kit_dio/lib/src/widgets/pluggable_state.dart

@@ -233,7 +233,7 @@ class _ResponseCardState extends State<_ResponseCard> {
     final Map<String, List<String>> map = _request.headers.map(
       (key, value) => MapEntry(
         key,
-        value is Iterable ? value.map((v) => v.toString()).toList() : [value],
+        value is Iterable ? value.map((v) => v.toString()).toList() : [value.toString()],
       ),
     );
     final Headers headers = Headers.fromMap(map);