Use InvalidUTF8ErrorHandler only for rails 4

In Rails 5 catches invalid UTF8 characters in querystring in a
params middleware, errors are handled by a params middleware and
raises a BadRequest exception. This means that these UTF8 errors
are not raised deeper in application stack and these can't also
be handled on application level.

If we would want to have custom handler for these errors, we would
have to create a new middleware and insert it before actionpack's
params middleware and rescue BadRequest exceptions there. But there
is no need to do this currently (see discussion on
https://gitlab.com/gitlab-org/gitlab-ce/issues/51908)

app/controllers/application_controller.rb

@@ -12,7 +12,9 @@ class ApplicationController < ActionController::Base
   include WorkhorseHelper
   include EnforcesTwoFactorAuthentication
   include WithPerformanceBar
-  include InvalidUTF8ErrorHandler
+  # this can be removed after switching to rails 5
+  # https://gitlab.com/gitlab-org/gitlab-ce/issues/51908
+  include InvalidUTF8ErrorHandler unless Gitlab.rails5?
 
   before_action :authenticate_sessionless_user!
   before_action :authenticate_user!

spec/controllers/application_controller_spec.rb

@@ -685,17 +685,28 @@ describe ApplicationController do
     end
 
     context 'html' do
+      subject { get :index, text: "hi \255" }
+
       it 'renders 412' do
-        get :index, text: "hi \255"
+        if Gitlab.rails5?
+          expect { subject }.to raise_error(ActionController::BadRequest)
+        else
+          subject
 
           expect(response).to have_gitlab_http_status(412)
           expect(response).to render_template :precondition_failed
         end
       end
+    end
 
     context 'js' do
+      subject { get :index, text: "hi \255", format: :js }
+
       it 'renders 412' do
-        get :index, text: "hi \255", format: :js
+        if Gitlab.rails5?
+          expect { subject }.to raise_error(ActionController::BadRequest)
+        else
+          subject
 
           json_response = JSON.parse(response.body)
 
@@ -704,6 +715,7 @@ describe ApplicationController do
         end
       end
     end
+  end
 
   context 'X-GitLab-Custom-Error header' do
     before do