"add" operator takes a list of ids

issue2923
(transplanted from 45f7e2edb709d497ef4ac917b54169ab17ba4c31)

invoice.py

@@ -95,7 +95,7 @@ class InvoiceLine:
             if field.startswith('analytic_account_'):
                 if vals[field]:
                     selection_vals.setdefault('accounts', [])
-                    selection_vals['accounts'].append(('add', vals[field]))
+                    selection_vals['accounts'].append(('add', [vals[field]]))
                 del vals[field]
         if vals.get('analytic_accounts'):
             Selection.write([Selection(vals['analytic_accounts'])],