error_page_spec.rb

def with_exceptions_app(enabled = true)
  org_show_detailed_exceptions = Rails.application.env_config['action_dispatch.show_detailed_exceptions']
  org_show_exceptions          = Rails.application.env_config['action_dispatch.show_exceptions']

  # This overrides the cached setting in Rails.application.config.consider_all_requests_local
  Rails.application.env_config['action_dispatch.show_detailed_exceptions'] = !enabled

  # Render templates instead of raising exceptions.
  Rails.application.env_config['action_dispatch.show_exceptions'] = enabled

  yield
ensure
  Rails.application.env_config['action_dispatch.show_detailed_exceptions'] = org_show_detailed_exceptions
  Rails.application.env_config['action_dispatch.show_exceptions']          = org_show_exceptions
end

it "shows an error page" do
  with_exceptions_app do
    get '/'
  end

  ...
end