Set the proper company_id on an imported partner

When a partner is being imported, the company id is not set, which leaves the choice to the system. While there is another solution proposed in #52, this makes things work until someone has the time to fix multicompany issues properly.
Maybe leave it to False if we do not pass one of the two `if`? Or raise an error.
It should not happen, but if it had to happen, it would have the company of the user that runs the jobs and it could mess around.