[FIX]stock: return of 'To Be Invoiced' picking should also be invoiced

When creating a return picking, the default invoice state is 'To Be Invoiced' if
returned picking was invoiced. However if the invoice of the picking has not
been generated yet (state '2binvoiced'), the return should also be invoiced.
Fixes #4002
Merge branch '7.0' of https://github.com/odoo/odoo into 7.0-win32