[FIX] account_voucher: wrong type in context

The type 'general' no longer exists on a account.voucher. As in general context, removed the default value on this menu.
Fixes #1261, it produces a traceback in 8.0 where the selection fields are less fault-tolerant.
Merge branch '7.0' of https://github.com/odoo/odoo into 7.0-win32