Mongoose not saving document

I've been working on a simple update function, that looks as follows:

exports.update = function update(req, res, next){

  User.findById(req.param('userId'))
    .exec(function (err, user) {
      if(err) return next(err);


      if(!user)
         return res.sendData(null, 404, lang.userNotFound);

      var info = req.param('info');

      // Get the update to be made, in JSON format
      if(info !== Object(info)) info = JSON.parse(info);

      user.info = setProperties(user.info, info);

      // console.log(user) gives the updated user object

      user.save(function(err, usr) {
        if (err) return next(err);

        // console.log(usr) shows the updated user object

        res.sendData();
      });
    });
};

function setProperties(object, update) {

  if(update!== Object(update))
    object = update;

  else {
    for (var prop in update) {
      if (object.hasOwnProperty(prop))
        object[prop] = setProperties(object[prop], update[prop]);
      else
        object[prop] = update[prop];
    }
  }
  return object;
}

But unfortunately, although everything seems to work, in my Database nothing changes. I do not get any errors. Could anyone shed some light on this mystery? Thanks!

For reference (if it is relevant), here is my Schema:

var UserSchema = new Schema({
  createdAt: {type: Date, default: Date.now},
  lastActivity: {type: Date, default: Date.now},
  info : {type : Schema.Types.Mixed},
 });