[FIX] payment_buckaroo: dynamic lang & test params

test flag is set according to acquirer environment
lang is set according to partner language
[FIX] payment_buckaroo: api expects iso culture as language param

IT looks like this is the code we use in partner lang, by replacing the '_' (underscore) by '-' (dash)
[FIX] purchase: supplier invoice and purchase order count

Value of purchase_order_count and supplier_invoice_count should be coherant with
the behaviour of the button on partner form: count records for the contact of
the company as well.
Fixes #4224
Merge branch '8.0' of https://github.com/odoo/odoo into 8.0-linux64