Fix render option to suppress deprecation warning
Fix render option to suppress deprecation warning

> DEPRECATION WARNING: `render :text` is deprecated because it does not
> actually render a `text/plain` response. Switch to `render plain:
> 'plain text'` to render as `text/plain`, `render html:
> '<strong>HTML</strong>'` to render as `text/html`, or `render body:
> 'raw'` to match the deprecated behavior and render with the default
> Content-Type, which is `text/html`.

Signed-off-by: Kenji Okimoto <7a4b90a0a1e6fc688adec907898b6822ce215e6c@clear-code.com>

fluent/fluentd-ui,fluent/fluentd-ui,fluent/fluentd-ui