Redsys: Check for non-3DS response before attempting 3DS parse

[britth]

Based on the Redsys docs, it seems that when you attempt 3DS but
3DS is not available, you will receive a direct response to your
request. Currently, we look under specific namespaces related to
the 3DS action step when performing 3DS, which would cause an error
if the service returned a direct response. This PR updates the
parse method to first look for the path //RETORNOXML/OPERACION
and if present (and successful), tries to parse the response.
Otherwise, we proceed to the 3DS specific parsing, or error
message if there was an issue.

Unfortunately, there's no way to test this in the Redsys sandbox,
but existing functionality appears in tact with this change.

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

Unit:
36 tests, 112 assertions, 0 failures, 0 errors, 0 pendings, 0 omissions, 0 notifications
100% passed

[bayprogrammer]

👍 Code looks good. Ran the tests locally. The redsys_sha356_test.rb and redsys_test.rb unit tests all pass for me. remote_redsys_sha256_test.rb passes for me as well, but remote_redsys_test.rb had some failures. Those fail for master as well, so they are not related to these updates to the RedsysGateway adapter.

[britth]

@bayprogrammer thanks for the review! Yeah, the remote_redsys_test file tests non-sha256 requests, which are technically deprecated, so we probably want to just remove those once we officially get rid of support in AM