Add closed_at field to issue CSV export

app/services/issues/export_csv_service.rb

@@ -41,6 +41,7 @@ module Issues
        'Due Date' => -> (issue) { issue.due_date&.to_s(:csv) },
        'Created At (UTC)' => -> (issue) { issue.created_at&.to_s(:csv) },
        'Updated At (UTC)' => -> (issue) { issue.updated_at&.to_s(:csv) },
+       'Closed At (UTC)' => -> (issue) { issue.closed_at&.to_s(:csv) },
        'Milestone' => -> (issue) { issue.milestone&.title },
        'Labels' => -> (issue) { @labels[issue.id].sort.join(',').presence }
       }

changelogs/unreleased-ee/sh-support-closed-at-csv.yml

+---
+title: Add closed_at field to issue CSV export
+merge_request:
+author:

spec/services/issues/export_csv_service_spec.rb

@@ -39,6 +39,7 @@ describe Issues::ExportCsvService, services: true do
                     due_date: DateTime.new(2014, 3, 2),
                     created_at: DateTime.new(2015, 4, 3, 2, 1, 0),
                     updated_at: DateTime.new(2016, 5, 4, 3, 2, 1),
+                    closed_at: DateTime.new(2017, 6, 5, 4, 3, 2),
                     labels: [feature_label, idea_label])
     end
 
@@ -101,6 +102,10 @@ describe Issues::ExportCsvService, services: true do
     specify 'updated_at' do
       expect(csv[0]['Updated At (UTC)']).to eq '2016-05-04 03:02:01'
     end
+
+    specify 'closed_at' do
+      expect(csv[0]['Closed At (UTC)']).to eq '2017-06-05 04:03:02'
+    end
   end
 
   context 'with minimal details' do