add an example of on_change usage

[florentx]

There's nothing specific for on_change events in ERPpeek.

The way to mimic the OpenERP client (6.1) is:

# on_change with ERPpeek for invoice creation

@step('this customer has {nb:d} invoices')
def impl(ctx, nb):
    assert_true(ctx.data['partner'])
    partner = ctx.data['partner']
    account_invoice_obj = model('account.invoice')
    for idx in xrange(1, nb + 1):
        # On change
        data = {
            'name': "Invoice # %d" % idx,
            'type': "out_invoice",
            'address_invoice_id': partner.address[0].id,
            'partner_id': partner.id,
        }
        rv = account_invoice_obj.onchange_partner_id([], 'out_invoice', partner.id, date.today(), False, False)
        data.update(rv['value'])
        inv = account_invoice_obj.create(data)
        if partner.contract_ids:
            inv.write({'contract_id': partner.contract_ids[0].id})
        ctx.data['invoice'] = inv

To be added to the documentation.