fix!: fix for the uk/gb country code case

New file:
* `api_off_tag_test.dart`: Tests about OffTagged enums.

Impacted file:
* `country_helper.dart`: case-insensitive search.

lib/src/utils/country_helper.dart

@@ -760,8 +760,12 @@ enum OpenFoodFactsCountry implements OffTagged {
   @override
   final String offTag;
 
-  /// Returns the first [OpenFoodFactsCountry] that matches the [offTag].
-  static OpenFoodFactsCountry? fromOffTag(final String? offTag) {
+  /// Returns the [OpenFoodFactsCountry] that matches the [offTag].
+  ///
+  /// Case-insensitive.
+  /// Special case: "uk" and "gb" both mean United Kingdom.
+  static OpenFoodFactsCountry? fromOffTag(String? offTag) {
+    offTag = offTag?.toLowerCase();
     // special case as we use 'uk' in off-dart
     if (offTag == 'gb') {
       return OpenFoodFactsCountry.UNITED_KINGDOM;

test/api_off_tag_test.dart

@@ -0,0 +1,25 @@
+import 'package:openfoodfacts/openfoodfacts.dart';
+import 'package:test/test.dart';
+
+/// Tests about [OffTagged] `enum`s.
+void main() {
+  test('$OpenFoodAPIClient country', () async {
+    const Map<String?, OpenFoodFactsCountry?> expectedCountries =
+        <String?, OpenFoodFactsCountry?>{
+      'fr': OpenFoodFactsCountry.FRANCE,
+      'FR': OpenFoodFactsCountry.FRANCE,
+      'fR': OpenFoodFactsCountry.FRANCE,
+      'Fr': OpenFoodFactsCountry.FRANCE,
+      'uk': OpenFoodFactsCountry.UNITED_KINGDOM,
+      'UK': OpenFoodFactsCountry.UNITED_KINGDOM,
+      'gb': OpenFoodFactsCountry.UNITED_KINGDOM,
+      'GB': OpenFoodFactsCountry.UNITED_KINGDOM,
+      null: null,
+      'this is not a country code': null,
+    };
+    for (final MapEntry<String?, OpenFoodFactsCountry?> entry
+        in expectedCountries.entries) {
+      expect(OpenFoodFactsCountry.fromOffTag(entry.key), entry.value);
+    }
+  });
+}