[FIX] mass_mailing: fix mass mailing sent date computation

Currently when you click on send now button or schedule button it will enter
current date as sent_date while the mailing list is still in queue. Actual
sent date will be the date when the mail is sent.

Related task ID : 46932

addons/mass_mailing/models/mass_mailing.py

@@ -646,7 +646,7 @@ class MassMailing(models.Model):
 
     @api.multi
     def put_in_queue(self):
-        self.write({'sent_date': fields.Datetime.now(), 'state': 'in_queue'})
+        self.write({'state': 'in_queue'})
 
     @api.multi
     def cancel_mass_mailing(self):
@@ -791,7 +791,7 @@ class MassMailing(models.Model):
             # auto-commit except in testing mode
             auto_commit = not getattr(threading.currentThread(), 'testing', False)
             composer.send_mail(auto_commit=auto_commit)
-            mailing.state = 'done'
+            mailing.write({'state': 'done', 'sent_date': fields.Datetime.now()})
         return True
 
     def convert_links(self):
@@ -825,4 +825,4 @@ class MassMailing(models.Model):
                 mass_mailing.state = 'sending'
                 mass_mailing.send_mail()
             else:
-                mass_mailing.state = 'done'
+                mass_mailing.write({'state': 'done', 'sent_date': fields.Datetime.now()})