Use pluck on DomainAllow.allowed_domains for export (#36019)

spec/requests/admin/export_domain_allows_spec.rb

@@ -3,9 +3,9 @@
 require 'rails_helper'
 
 RSpec.describe 'Admin Export Domain Allows' do
-  describe 'POST /admin/export_domain_allows/import' do
-    before { sign_in Fabricate(:admin_user) }
+  before { sign_in Fabricate(:admin_user) }
 
+  describe 'POST /admin/export_domain_allows/import' do
     it 'gracefully handles invalid nested params' do
       post import_admin_export_domain_allows_path(admin_import: 'invalid')
 
@@ -13,4 +13,28 @@ RSpec.describe 'Admin Export Domain Allows' do
         .to redirect_to(admin_instances_path)
     end
   end
+
+  describe 'GET /admin/export_domain_allows/export.csv' do
+    before do
+      Fabricate(:domain_allow, domain: 'good.domain')
+      Fabricate(:domain_allow, domain: 'better.domain')
+    end
+
+    it 'returns CSV response with instance domain values' do
+      get export_admin_export_domain_allows_path(format: :csv)
+
+      expect(response)
+        .to have_http_status(200)
+      expect(response.body)
+        .to eq(domain_allows_csv_file)
+      expect(response.media_type)
+        .to eq('text/csv')
+    end
+  end
+
+  private
+
+  def domain_allows_csv_file
+    File.read(File.join(file_fixture_path, 'domain_allows.csv'))
+  end
 end