Since Ruby's default behavior without locale configuration such as `LANG` is default to ASCII-8BIT, default should be `ASCII-8BIT`, since it may break other user's page build.

We can discuss whether defaults to be `utf-8` or not later. This discussion can be endless. Thus, I recommend to stick with current behavior, unless clearly configured with `encoding` config. This allows us to configure to use `utf-8` while keeping other page builds happy.