bill task work selects wrong receivable account in multi-company context

Hello,

When running Bill Task Work, it is possible to create an invoice where the partner account (field account_id) is from a different company than the invoice.

This one applies to 6.1 and trunk.
It is a variant of lp:1047826.

How to reproduce:
1. install trunk and accounting module and hr_timesheet_invoice module
2. create 2 companies A and B with diferrent charts of account
3. create a 3rd company M as partent of A and B
4. set set user preferences to company M
5. create a partner
6. create an analytic account named Project1 for company A with invoice factor = 100%
7. create timesheets on Project1
8. run bill task work
9. you get a draft invoice for company A (the account's company), this is correct
10. if you click on the receivable account to see the details, you see it is the receivable account of company M -> this is not correct.

I'll attach a merge proposal.

-sbi