static removeHiddenProperties(obj) {
    for (var key in obj) {
      if (Object.prototype.hasOwnProperty.call(obj, key)) {
        // Regexp comes from Parse.Object.prototype.validate
        if (key !== '__type' && !/^[A-Za-z][0-9A-Za-z_]*$/.test(key)) {
          delete obj[key];
        }
      }
    }
  }
Removes all "_" prefixed properties from an object, except "__type"
@param {Object} obj An object.