[FIX] account_asset: allow deferred revenue on one month
[FIX] account_asset: allow deferred revenue on one month

One could want to use deferred revenue for revenues invoiced in February
but the services are provided in June (e.g.)

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