HttpHeaders

packages/kiota_abstractions/lib/src/case_insensitive_map.dart

@@ -109,7 +109,13 @@ class CaseInsensitiveMap<K extends String, V> implements Map<K, V> {
 
   @override
   V putIfAbsent(K key, V Function() ifAbsent) {
-    return _contents.putIfAbsent(normalizeKey(key), ifAbsent);
+    final normalizedKey = normalizeKey(key);
+
+    return _contents.putIfAbsent(normalizedKey, () {
+        _originalKeys[normalizedKey] = key;
+
+        return ifAbsent();
+    });
   }
 
   @override

packages/kiota_abstractions/lib/src/extensions/request_information_extensions.dart

@@ -13,7 +13,7 @@ extension RequestInformationExtensions on RequestInformation {
 
   /// Vanity method to add headers to the request.
   void addHeaders(HttpHeaders headers) {
-    headers.addAll(headers);
+    this.headers.addAll(headers);
   }
 
   /// Vanity method to add query parameters to the request.

packages/kiota_abstractions/test/case_insensitive_map_test.dart

@@ -0,0 +1,20 @@
+// ignore_for_file: avoid_redundant_argument_values
+
+import 'package:kiota_abstractions/kiota_abstractions.dart';
+import 'package:test/test.dart';
+
+void main() {
+  group('CaseInsensitiveMap', () {
+    test('putIfAbsent', () {
+      final headers = HttpHeaders()
+        ..putIfAbsent('key', () => {'value'})
+        ..map((name, values) {
+          return MapEntry(name, values.join(', '));
+        });
+      expect(
+        headers['key'],
+        {'value'},
+      );
+    });
+  });
+}

packages/kiota_abstractions/test/request_information_test.dart

@@ -226,5 +226,17 @@ void main() {
       // Assert
       expect(testRequest.uri.toString(), 'http://localhost/first,second');
     });
+
+    test('Add headers', () {
+      // Arrange
+      final testRequest = RequestInformation();
+      final headers = HttpHeaders()..putIfAbsent('key', () => {'value'});
+
+      // Act
+      testRequest.addHeaders(headers);
+
+      // Assert
+      expect(testRequest.headers['key'], {'value'});
+    });
   });
 }