Use real Many2Many for pay_invoice.ask

invoice.py

@@ -1963,7 +1963,7 @@ class PayInvoiceAsk(WizardOSV):
     currency_digits = fields.Integer('Currency Digits', readonly=True,
             on_change_with=['currency'])
     lines_to_pay = fields.Char(string='Lines to Pay', size=None)
-    lines = fields.One2Many('account.move.line', 'ham', 'Lines',
+    lines = fields.Many2Many('account.move.line', None, None, None, 'Lines',
             domain="[('id', 'in', eval(lines_to_pay)), " \
                     "('reconciliation', '=', False)]",
             states={

invoice.xml

@@ -897,7 +897,7 @@ this repository contains the full copyright notices and license terms. -->
                     <label name="currency"/>
                     <field name="currency"/>
                     <separator name="lines" colspan="4"/>
-                    <field name="lines" colspan="4" widget="many2many">
+                    <field name="lines" colspan="4">
                         <tree string="Lines to Pay">
                             <field name="maturity_date"/>
                             <field name="debit" sum="Debit"/>