[FIX] opportunity: salesperson not in followers

Previously, when updating subscribers of a mail.thread, the default_{key}
arguments could override a {key} in the values.

e.g: for an new opportunity, the salesperson was overrided by the default user_id

This fix gives the priority to the values over the default_ in context.

opw-631741

addons/mail/mail_thread.py

@@ -263,7 +263,7 @@ class mail_thread(osv.AbstractModel):
         # auto_subscribe: take values and defaults into account
         create_values = dict(values)
         for key, val in context.iteritems():
-            if key.startswith('default_'):
+            if key.startswith('default_') and key[8:] not in create_values:
                 create_values[key[8:]] = val
         self.message_auto_subscribe(cr, uid, [thread_id], create_values.keys(), context=context, values=create_values)