Make sure that we set `env["action_dispatch.show_exceptions"]`

This has been used by `show_exception` middleware even the setting wasn't get passed to `env` hash.

railties/lib/rails/application.rb

@@ -122,7 +122,8 @@ module Rails
       @env_config ||= super.merge({
         "action_dispatch.parameter_filter" => config.filter_parameters,
         "action_dispatch.secret_token" => config.secret_token,
-        "action_dispatch.asset_path" => nil
+        "action_dispatch.asset_path" => nil,
+        "action_dispatch.show_exceptions" => config.action_dispatch.show_exceptions
       })
     end
 

railties/test/application/configuration_test.rb

@@ -318,5 +318,20 @@ module ApplicationTests
 
       assert ActionView::Resolver.caching?
     end
+
+    test "config.action_dispatch.show_exceptions is sent in env" do
+      make_basic_app do |app|
+        app.config.action_dispatch.show_exceptions = true
+      end
+
+      class ::OmgController < ActionController::Base
+        def index
+          render :text => env["action_dispatch.show_exceptions"]
+        end
+      end
+
+      get "/"
+      assert_equal 'true', last_response.body
+    end
   end
 end