Add test

Author: bigsolom

test/invoice_test.rb

@@ -588,5 +588,12 @@ def test_negative_de_invoice_against_schematron_2
       end
       assert_equal [], errors
     end
+
+    def test_invoice_object_extensions
+      invoice = make_de_invoice
+      xml = invoice.to_xml(version: 2)
+
+      assert_match(/<ram:PaymentReference>#{invoice.payment_reference}<\/ram:PaymentReference>/, xml)
+    end
   end
 end