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.
> and everything stays as it was (setting it according the running user)

Yes. If it does not pass either 'if', this is an exceptional situation, and in that case, assigning the company of the running user would be worse (example: the company of the connector user is "the holding", the regular users all connects to the children companies, no one will be able to see this customer), IMO, than setting no company at all.


> Or are you suggesting returning {'company_id': False} in that case?

Yes. 
Or we could also claim that it should never happen and we raise a MappingError