This commit targets to fix two errors:

1.- When trying to create the EDI document for
an invoice that has a sale order related to it, when using the Addenda Autozone a traceback appeared.

### Cause:
In attribute t-att-PODATE for the Autozone Addenda we are sending a datetime object
to the strptime function of datetime which in reality it needs to receive a string
object instead. This raises a TypeError

### Solution:
Since the objective is to set a string date, we convert the date_order into a string date
using strftime function from datetime module

### Steps to reproduce:

    - Install 'Sales' application and 'l10n_mx_edi' module
    - In a company with mexican localization selected go to Sales > Quotations and create a new Order
    - Select a contact with the Addenda Autozone selected
    - Confirm it and create the corresponding invoice
    - Confirm the invoice and generate the CFDI
***
2.- When creating an invoice from multiple sale orders with contact with Addenda Autozone selected, a traceback is raised

### Steps to reproduce:

    - Install 'Sales' application and 'l10n_mx_edi' module
    - In a company with mexican localization selected go to Sales > Quotations
    - Create two orders at least with a contact with Addenda Autozone selected and confirmed them
    - In tree view select created orders and click on Create Invoices
    - Go to Created Invoice and confirm it and generate the CFDI

### Cause:
In attribute t-att-PODATE for the Autozone Addenda is not expecting a recordset when initializing the
value of sale_id, this causes a ValueError: expected singleton error when retrieving value of date_order
when we create a invoice for multiple sale orders

### Solution:
Initialize sale_id with the first order retrieved to use it