Convert helper output_buffer to string

Starting from Rails 7.1, `ActionView::TestCase#output_buffer` uses
`ActionView::OutputBuffer` rather than `ActiveSupport::SafeBuffer`.

Due to the slightly different behavior, before comparing it to a
string we we need first to convert it.

See rails/rails@532e39a

core/spec/helpers/base_helper_spec.rb

@@ -86,7 +86,7 @@
       expect(html.css(".notice").text).to eq("ok")
       expect(html.css(".foo").text).to be_empty
       expect(html.css(".bar").text).to be_empty
-      expect(helper.output_buffer).to eq("<div class=\"flash notice\">ok</div>")
+      expect(helper.output_buffer.to_s).to eq("<div class=\"flash notice\">ok</div>")
     end
   end