message: API: Fix Course toObject courseAdmin id extraction
diff: @@ -107,9 +107,7 @@ const courseSchema = new mongoose.Schema({
  ret._id = ret._id.toString();
  }
  - if (ret.hasOwnProperty('courseAdmin') && ret.courseAdmin !== null && (ret.courseAdmin instanceof ObjectID)) {
  - ret.courseAdmin = ret.courseAdmin.toString();
  - }
  + ret.courseAdmin = extractSingleMongoId(ret.courseAdmin);
  ret.hasAccessKey = false;
  if (ret.accessKey) {
  ret.hasAccessKey = true;
  