Litle: fixed refund with custom billing descriptor

[softbrada]

Currently, when doing a refund with a custom billing descriptor the response I get this error

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>
<litleOnlineResponse xmlns="http://www.litle.com/schema" version="9.14" response="1" message="Error validating xml data against the schema: cvc-complex-type.2.4.a: Invalid content was found starting with element 'customBilling'. One of '{&quot;http://www.litle.com/schema&quot;:transaction, &quot;http://www.litle.com/schema&quot;:recurringTransaction}' is expected." />

I checked all other versions of schema, higher versions also, and the problem is that the customBilling element is currently a sibling with the credit or echeckCredit element , but according to the schema it should be inside the credit or echeckCredit

The other methods add_echeck_purchase_params and add_credit_params already handle the custom billing descriptors correctly

[github-actions[bot]]

To provide a cleaner slate for the maintenance of the library, this PR/Issue is being labeled stale after 60 days without activity. It will be closed in 14 days unless you comment with an update regarding its applicability to the current build. Thank you!