[FIX] mail: return str instead of bytes when post processing attachment

Steps to reproduce:

  On Odoo:
  - Install `Documents` module
  - Go to `Settings` and set a Custom Email Server(ex. `mydomain.com`)
  - Ensure an alias exist for the model `document.document` with
    `inbox-financial` as alias name

  In mail client:
  - Send a mail with an image in the body to the following email:
    inbox-financial@mydomain.com

Issue:

  Mail not received (traceback in logs)

Cause:

  Since we use the email alias `inbox-financial`, we process the mail
  through the `document.document` model where we have an override of the
  `_message_post_after_hook` method that is called after that the
  `msg_values` values are post-processed and where we do another
  message_post() for the new document create for the attachment (in
  this case, an image).

  https://github.com/odoo/enterprise/blob/2c3596e4e18c201809558d3ea878b141e366a027/documents/models/document.py#L305

  During the parsing, the mail values are updated through the
  `_process_attachments_for_post` method:

  https://github.com/odoo/odoo/blob/6c0d2d7a9d44459f3e09a38bd80ef9b018e8c946/addons/mail/models/mail_thread.py#L1881-L1904

  On posting the first time, the original type of the `body` value is a
  `str`, but the post-processed value (because there is some CIDS in the
  body) is of type `bytes` (because using `encoding='UTF-8'` with
  `lxml.html.tostring`).

  https://github.com/odoo/odoo/blob/510a997017a9cbe14522a0013a578f6d1d9b257a/addons/mail/models/mail_thread.py#L2209

  Then in the `_message_post_after_hook` we call post again on the
  newly created document record by using post-processed value of body
  who is of type `bytes`.

  On posting the second time (for the document record), the `body`
  value is of type `bytes` and when checking if the body is empty with
  `is_html_empty` that received a string as param, an error is
  raised.

Solution:

  Use `encoding='unicode'` to return a string instead of bytes.

  https://lxml.de/api/lxml.etree-module.html#tostring

ENT PR : https://github.com/odoo/enterprise/pull/42653



opw-3273583

closes odoo/odoo#135021

X-original-commit: f910987c
Signed-off-by: Thibault Delavallee (tde) <tde@openerp.com>