[FIX] mail_thread inheritance: add api.returns on override of message_post

As message_post is defined on mail.thread abstract class its api.returns
is not automatically added on overridden versions of the method. This
commit add missing api.returns on all overrides of the method.

addons/hr/models/res_users.py

@@ -31,6 +31,7 @@ class User(models.Model):
         return self.env['hr.employee'].with_context(ctx).search([('user_id', '=', self.id)])
 
     @api.multi
+    @api.returns('self', lambda value: value.id)
     def message_post(self, **kwargs):
         """ Redirect the posting of message on res.users to the related employees.
             This is done because when giving the context of Chatter on the

addons/mail/models/res_users.py

@@ -77,6 +77,7 @@ class Users(models.Model):
         return self.partner_id.id
 
     @api.multi
+    @api.returns('self', lambda value: value.id)
     def message_post(self, **kwargs):
         """ Redirect the posting of message on res.users as a private discussion.
             This is done because when giving the context of Chatter on the

addons/website_blog/models/website_blog.py

@@ -33,6 +33,7 @@ class Blog(models.Model):
         return res
 
     @api.multi
+    @api.returns('self', lambda value: value.id)
     def message_post(self, parent_id=False, subtype=None, **kwargs):
         """ Temporary workaround to avoid spam. If someone replies on a channel
         through the 'Presentation Published' email, it should be considered as a

addons/website_forum/models/forum.py

@@ -799,6 +799,7 @@ class Post(models.Model):
         return groups
 
     @api.multi
+    @api.returns('self', lambda value: value.id)
     def message_post(self, message_type='notification', subtype=None, **kwargs):
         question_followers = self.env['res.partner']
         if self.ids and message_type == 'comment':  # user comments have a restriction on karma

addons/website_slides/models/slides.py

@@ -165,6 +165,7 @@ class Channel(models.Model):
         return res
 
     @api.multi
+    @api.returns('self', lambda value: value.id)
     def message_post(self, parent_id=False, subtype=None, **kwargs):
         """ Temporary workaround to avoid spam. If someone replies on a channel
         through the 'Presentation Published' email, it should be considered as a