[FIX] mass_mailing: save codeview to style-inline

Scenario:

- edit a mass mailing while in debug mode
- switch to codeview
- do a change
- click on save

Issue:

The body_arch is updated (making it seems like our change are taken into
account), but the body_html is not and the outdated version will be sent.

Solution:

With this change, when we commit the changes and have the codeview
opened, we will first update the wysiwyg editable to the current
codeview value, before updating body_html.

related to opw-3103549

closes odoo/odoo#119206

Signed-off-by: Nicolas Lempereur (nle) <nle@odoo.com>