[FIX] mail: fix method _get_default_author to return a recordset instead of an id

addons/mail/models/mail_message.py

@@ -47,7 +47,7 @@ class Message(models.Model):
 
     @api.model
     def _get_default_author(self):
-        return self.env.user.partner_id.id
+        return self.env.user.partner_id
 
     # content
     subject = fields.Char('Subject')

addons/mail/models/mail_thread.py

@@ -1467,7 +1467,7 @@ class MailThread(models.AbstractModel):
         # 0: Find the message's author, because we need it for private discussion
         author_id = kwargs.get('author_id')
         if author_id is None:  # keep False values
-            author_id = self.env['mail.message']._get_default_author()
+            author_id = self.env['mail.message']._get_default_author().id
 
         # 1: Handle content subtype: if plaintext, converto into HTML
         if content_subtype == 'plaintext':