fix: 829 - no more crash if the user name is not ASCII/ISO-8859-1 (#830)

Impacted files:
* `http_helper.dart`: safer `From` http header.
* `user_management_test_test_env.dart`: new test if the login crashes with a non-ASCII/ISO-8859-1 username

lib/src/utils/http_helper.dart

@@ -96,7 +96,7 @@ class HttpHelper {
       }
     }
 
-    http.Response response = await http.post(
+    return http.post(
       uri,
       headers: _buildHeaders(
         user: user,
@@ -105,7 +105,6 @@ class HttpHelper {
       ),
       body: addUserAgentParameters(body),
     );
-    return response;
   }
 
   static const String userInfoForTest = 'off:off';
@@ -214,7 +213,9 @@ class HttpHelper {
     headers.addAll({
       'Accept': 'application/json',
       'User-Agent': OpenFoodAPIConfiguration.userAgent!.toValueString(),
-      'From': OpenFoodAPIConfiguration.getUser(user)?.userId ?? FROM,
+      'From': _getSafeString(
+        OpenFoodAPIConfiguration.getUser(user)?.userId ?? FROM,
+      ),
     });
 
     final bool isTestModeActive = uriHelper.isTestMode;
@@ -254,4 +255,27 @@ class HttpHelper {
       rethrow;
     }
   }
+
+  /// Returns true if the [input] string is ISO-8859-1 encoded.
+  static bool _isIso88591(final String input) {
+    try {
+      latin1.encode(input);
+      return true;
+    } catch (e) {
+      return false;
+    }
+  }
+
+  /// Returns the [input] string if ISO-8859-1 encoded, or a safer version of it
+  ///
+  /// Used for HTTP headers, that do not accept any other charset.
+  /// Here instead of encoding systematically, we keep ISO-8859-1 inputs and
+  /// only encode the other "problematic" inputs.
+  /// cf. https://github.com/openfoodfacts/openfoodfacts-dart/issues/829
+  static String _getSafeString(final String input) {
+    if (_isIso88591(input)) {
+      return input;
+    }
+    return base64Encode(utf8.encode(input));
+  }
 }

test/user_management_test_test_env.dart

@@ -61,6 +61,15 @@ void main() {
       expect(status?.successful, false);
       expect(status?.statusVerbose, 'user not signed-in');
     });
+
+    test('Login with problematic charset', () async {
+      final LoginStatus? status = await OpenFoodAPIClient.login2(
+        User(userId: 'លីវយី', password: ''),
+        uriHelper: uriHelper,
+      );
+      expect(status?.successful, false);
+      expect(status?.statusVerbose, 'user not signed-in');
+    });
   });
 }