[FIX] project_issue_sheet: make invoiceable field autofield for worklogs

When setting a worklog on a project_issue, the field to_invoice is prefilled with on_change_account_id based on the contract settings. As the field was not present on the list view, the information was lost and every worklog was not written as invoiceable, even if enabled on the contract. opw 609082.
Merge branch '7.0' of https://github.com/odoo/odoo into 7.0-win32