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 <okimoto@clear-code.com>

spec/controllers/application_controller_spec.rb

@@ -6,7 +6,7 @@ describe DummyController do
   controller DummyController do
     skip_before_action :login_required
     def index
-      render :text => "Hello World"
+      render plain: "Hello World"
     end
   end