Fix bug with `clear_user` and add `default_user_roles`

- Changed call to `remove_instance_variables` on `User` to use the
  `send` method since it's a private method.

- Added `default_user_roles` so a default usage
  of extending `RailsAttrEnum` and calling `attr_enum` could be
  established. This allows other future specs that depend on that
  default usage to not be messed up by the user_spec.rb file which
  alters `RailsAttrEnum` usage in `User` for different tests.
bump to v0.0.6