Redsys: Properly escape special characters in 3DS requests

[britth]

In some Redsys 3DS requests, the API will return SIS0042, which
signifies an error calculating the signature. After some testing,
this appears to be caused by special characters in various fields
of the request (in particular the DS_MERCHANT_TITULAR field).
Non-3DS requests escape the entire xml_request_from response, but
after testing the same approach for 3DS, that doesn't quite work
(you'll still end up with an encoding error after the first round of
3ds-related api requests). This PR updates to escape the cardholder
name when the transaction calls for 3DS to prevent such errors. It
also does the same thing for the description field as that seemed
like the most likely other place where a special character could
end up. I feel like there's probably a better way to do this, so if
you have another idea, please feel free to suggest!

If you'd like to manually test, try issuing a 3ds purchase request
against this branch, using a name or description that includes a
special character - the transaction should succeed. Then try issuing
a request using a special character against the active merchant master
branch, and it should fail.

Unit:
38 tests, 122 assertions, 0 failures, 0 errors, 0 pendings, 0 omissions, 0 notifications
100% passed

Remote:
22 tests, 70 assertions, 0 failures, 0 errors, 0 pendings, 0 omissions, 0 notifications
100% passed

[chinhle23-zz]

Confirmed through external testing that SIS0042 ERROR is returned when there is a special character in the cardholder's name with 3DS requests on master. This branch fixes this error and 3DS transactions are successful with special characters in the cardholder's name. Other than the rubocop errors, LGTM 👍