[FIX] mail: UTF-8 text/xml attachment and omitted charset

When parsing an email containing an xml attachment, the `email` python
module will decode the base64 attachment using the charset or ascii if
the charset is missing.

In some cases, the payload is in UTF-8 but the charset is omitted. This
results in replacement characters for the non ASCII characters.

The solution is to force the charset to UTF-8, since it is a superset of
ASCII that should not be a problem.

NB1: Omitting the charset for text/xml is not recommended. See the RFC
(section 6.4): https://www.ietf.org/rfc/rfc2376.txt



opw-3144519

closes odoo/odoo#125628

Signed-off-by: Julien Castiaux (juc) <juc@odoo.com>