Disable display of detailed exceptions

`@allow-rescue` permits rescuing exceptions by setting
`action_dispatch.show_exceptions` to true in the env hash. However,
this will render the detailed error page and not the public error page
you'd encounter in production.

Given that the feature enables testing of error pages, and seems to be
intended to mirror production behaviour as per the comment on
https://github.com/cucumber/cucumber-rails/blob/master/lib/generators/cucumber/install/templates/support/_rails_each_run.rb.erb#L6-L14,
this PR sets `action_dispatch.show_detailed_exceptions` to false, thus
enabling the public error page to be rendered.

features/allow_rescue.feature

@@ -23,20 +23,25 @@ Feature: Allow Cucumber to rescue exceptions
       Feature: posts
         @allow-rescue
         Scenario: See posts
-          When I look at the posts
+        When I look at the posts
+        Then I should see the public error page
       """
     And I write to "features/step_definitions/posts_steps.rb" with:
       """
       When /^I look at the posts$/ do
         visit '/posts'
       end
+      Then /^I should see the public error page$/ do
+        expect(page).to have_content "We're sorry, but something went wrong. \
+          If you are the application owner check the logs for more information."
+      end
       """
     And I run `bundle exec rake db:migrate`
     And I run `bundle exec cucumber`
     Then the feature run should pass with:
       """
       1 scenario (1 passed)
-      1 step (1 passed)
+      2 steps (2 passed)
       """
 
   Scenario: Don't allow rescue

lib/cucumber/rails/action_controller.rb

@@ -7,6 +7,7 @@ class ActionDispatch::ShowExceptions
 
   def call(env)
     env['action_dispatch.show_exceptions'] = !!ActionController::Base.allow_rescue
+    env['action_dispatch.show_detailed_exceptions'] = !ActionController::Base.allow_rescue
     __cucumber_orig_call__(env)
   end
 end