Fix ProjectCacheWorker for plain READMEs

The ProjectCacheWorker refreshes cache periodically, but it runs outside Rails
context. So include the ActionView helpers so the `content_tag` method is
available.

app/helpers/markup_helper.rb

 require 'nokogiri'
 module MarkupHelper
+  include ActionView::Helpers::TagHelper
+  include ActionView::Context
+
   def plain?(filename)
     Gitlab::MarkupHelper.plain?(filename)
   end

spec/workers/project_cache_worker_spec.rb

@@ -45,6 +45,18 @@ describe ProjectCacheWorker do
         worker.perform(project.id, %w(readme))
       end
+
+      context 'with plain readme' do
+        it 'refreshes the method caches' do
+          allow(MarkupHelper).to receive(:gitlab_markdown?).and_return(false)
+          allow(MarkupHelper).to receive(:plain?).and_return(true)
+
+          expect_any_instance_of(Repository).to receive(:refresh_method_caches).
+                                                  with(%i(readme)).
+                                                  and_call_original
+          worker.perform(project.id, %w(readme))
+        end
+      end
     end
   end