[FIX] base: specify the charset the the SMTP headers

The emails sent via Odoo reveive a poor score on various spam tools,
the reason being the missing `MIME-Version` message header. This header
inform what MIME version is in use in the message. Even if there is only
one standardized version of MIME (1.0), the header is mandatory.

Citing [rfc2045] "MIME: Format of Internet Message Bodies" section 4:

> the MIME-Version header field is required at the top level of a message.

Setting the charset on the envelop forces the EmailMessage API to also
include the missing `MIME-Version` header.

[rfc2045]: https://tools.ietf.org/html/rfc2045#section-4



Task 2393865

closes odoo/odoo#64651

Signed-off-by: Julien Castiaux <Julien00859@users.noreply.github.com>

odoo/addons/base/models/ir_mail_server.py

@@ -288,6 +288,8 @@ class IrMailServer(models.Model):
         body = body or u''
 
         msg = EmailMessage(policy=email.policy.SMTP)
+        msg.set_charset('utf-8')
+
         if not message_id:
             if object_id:
                 message_id = tools.generate_tracking_message_id(object_id)