Use the correct class for the CsvExport mailer spec

spec/mailers/emails/csv_export_spec.rb

@@ -9,7 +9,7 @@ describe Emails::CsvExport do
     let(:user) { create(:user) }
     let(:empty_project) { create(:empty_project, path: 'myproject') }
     let(:export_status) { { truncated: false, rows_expected: 3, rows_written: 3 } }
-    subject { described_class.issues_csv_email(user, empty_project, "dummy content", export_status) }
+    subject { Notify.issues_csv_email(user, empty_project, "dummy content", export_status) }
     let(:attachment) { subject.attachments.first }
 
     it 'attachment has csv mime type' do