Do not copy deferrals when copying account

issue2892
review580004
(transplanted from 75313e5b3c7a767402bcc9c43a19ccef89af15ca)

account.py

@@ -843,6 +843,7 @@ class Account(ModelSQL, ModelView):
             default = {}
         default['left'] = 0
         default['right'] = 0
+        default.setdefault('deferrals', [])
         new_accounts = super(Account, cls).copy(accounts, default=default)
         cls._rebuild_tree('parent', None, 0)
         return new_accounts