[FIX] sale_timesheet: fix traceback in write
[FIX] sale_timesheet: fix traceback in write

When calling `write` with more than one id, `_get_timesheet_cost`
generates a traceback since it uses `self.is_timesheet`.

hip-odoo/odoo,hip-odoo/odoo,bplancher/odoo,sysadminmatmoz/OCB,sysadminmatmoz/OCB,Elico-Corp/odoo_OCB,ygol/odoo,microcom/odoo,dfang/odoo,microcom/odoo,hip-odoo/odoo,storm-computers/odoo,bplancher/odoo,sysadminmatmoz/OCB,sysadminmatmoz/OCB,ygol/odoo,dfang/odoo,laslabs/odoo,sysadminmatmoz/OCB,bplancher/odoo,sysadminmatmoz/OCB,laslabs/odoo,bplancher/odoo,hip-odoo/odoo,storm-computers/odoo,laslabs/odoo,ygol/odoo,storm-computers/odoo,ygol/odoo,storm-computers/odoo,hip-odoo/odoo,hip-odoo/odoo,microcom/odoo,ygol/odoo,laslabs/odoo,bplancher/odoo,sysadminmatmoz/OCB,storm-computers/odoo,microcom/odoo,bplancher/odoo,Elico-Corp/odoo_OCB,Elico-Corp/odoo_OCB,Elico-Corp/odoo_OCB,dfang/odoo,dfang/odoo,microcom/odoo,dfang/odoo,Elico-Corp/odoo_OCB,laslabs/odoo,Elico-Corp/odoo_OCB,laslabs/odoo,microcom/odoo,dfang/odoo,ygol/odoo,ygol/odoo,storm-computers/odoo