Products.CAS4PAS doesn't handle validation responses with an XML-declaration correctly

We recently tried to update our CAS-infrastructure. Before the update, a validation response started like this:

After the update, a validation started like this:

Notice the XML-declaration (` 0:
# We have an XML response (CAS architecture 2.0)

Start in CASAuthHelper.py at the validation-response handling shown in the issue, and inspect how the first response line is used to distinguish CAS v1 from v2. Reproduce a validation response beginning with an XML declaration, then verify that the subsequent CAS service response is recognized and authentication succeeds.

Written by the indexing model from the issue text.