[FIX] mass_mailing: str/bytes confusion thing

qweb returns bytes, which would be stored in a dict's `body_html`,
which would then be stringified *but not decoded*.

Running with `-b` this gets flagged, it's probably better to fill the
dict with what's actually expected (text).

addons/mass_mailing/tests/test_mailing_internals.py

@@ -49,7 +49,7 @@ class TestMassMailValues(MassMailCommon):
         })
 
         mail_values = composer.get_mail_values([recipient.id])
-        body_html = str(mail_values[recipient.id]['body_html'])
+        body_html = mail_values[recipient.id]['body_html']
 
         self.assertIn('<!DOCTYPE html>', body_html)
         self.assertIn('<head>', body_html)

addons/mass_mailing/wizard/mail_compose_message.py

@@ -77,7 +77,7 @@ class MailComposeMessage(models.TransientModel):
                     'email': mail_to or mail_values.get('email_to'),
                 }
                 if mail_values.get('body_html') and mass_mail_layout:
-                    mail_values['body_html'] = mass_mail_layout._render({'body': mail_values['body_html']}, engine='ir.qweb', minimal_qcontext=True)
+                    mail_values['body_html'] = mass_mail_layout._render({'body': mail_values['body_html']}, engine='ir.qweb', minimal_qcontext=True).decode()
                 # propagate ignored state to trace when still-born
                 if mail_values.get('state') == 'cancel':
                     trace_vals['ignored'] = fields.Datetime.now()