fix(dart_frog): update Response.json headers to <String, Object> (#331)

Author: felangel

packages/dart_frog/lib/src/response.dart

@@ -36,7 +36,7 @@ class Response {
   Response.json({
     int statusCode = 200,
     Object? body = const <String, dynamic>{},
-    Map<String, String> headers = const <String, String>{},
+    Map<String, Object> headers = const <String, Object>{},
   }) : this(
           statusCode: statusCode,
           body: body != null ? jsonEncode(body) : null,

packages/dart_frog/test/src/response_test.dart

@@ -28,7 +28,15 @@ void main() {
     test('has correct headers', () {
       const headers = <String, String>{'foo': 'bar'};
       final response = Response(headers: headers);
-      expect(response.headers['foo'], equals(headers['foo']));
+      expect(response.headers['foo'], equals('bar'));
+    });
+
+    test('has correct headers when a key has multiple values', () {
+      const headers = <String, Object>{
+        'foo': ['bar', 'baz'],
+      };
+      final response = Response(headers: headers);
+      expect(response.headers['foo'], equals('bar,baz'));
     });
 
     group('copyWith', () {