[FIX] account: default partner format

When creating a new account.move.line, the computation of the default values should accept both `id` and `(id, name)` format.
The key partner_id may be containing a tuple so browsing should only be done on the first element of the tuple.
Fixes #3386
Merge branch '8.0' of https://github.com/odoo/odoo into 8.0-linux64