erp5_accounting: Add also string_index to the list of copied properties

in API script AccountingTransaction_createReversalTransaction
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Status Job ID Name Coverage
  External
passed ERP5.CodingStyleTest-Master

00:50:21

failed ERP5.UnitTest-Master

02:10:00

failed ERP5.UnitTest-Master.Medusa

01:29:46

passed SlapOS.Eggs.UnitTest-Master.Python2

00:33:26

passed SlapOS.Eggs.UnitTest-Master.Python3

00:32:03

failed ERP5.UnitTest-Master

02:03:19

passed SlapOS.Eggs.UnitTest-Master.Python2

00:09:25

passed SlapOS.Eggs.UnitTest-Master.Python3

00:09:07