add request store caching to favicon

lib/gitlab/favicon.rb

@@ -24,7 +24,7 @@ module Gitlab
       private
 
       def appearance
-        Appearance.current || Appearance.new
+        RequestStore.store[:appearance] ||= (Appearance.current || Appearance.new)
       end
 
       def appearance_favicon

spec/lib/gitlab/favicon_spec.rb

@@ -1,6 +1,6 @@
 require 'rails_helper'
 
-RSpec.describe Gitlab::Favicon do
+RSpec.describe Gitlab::Favicon, :request_store do
   describe '.default' do
     it 'defaults to favicon.ico' do
       allow(Rails).to receive(:env).and_return(ActiveSupport::StringInquirer.new('production'))