[FIX] account_invoice_tax_required: Avoid gaps in sequence

Using `invoice_validate` method for validating invoice, makes the call to the sequence
to be done before, and although rollbacking the transaction, the next number is already
assigned and then you may have gaps in your numbering.

Thus, we do the check before this, on the first executed method.

account_invoice_tax_required/__manifest__.py

@@ -4,7 +4,7 @@
 # License AGPL-3.0 or later (http://www.gnu.org/licenses/agpl).
 {
     'name': "Tax required in invoice",
-    'version': "10.0.1.0.1",
+    'version': "10.0.1.0.2",
     "author": "Camptocamp,Tecnativa,Odoo Community Association (OCA)",
     'website': "http://www.camptocamp.com",
     'category': "Localisation / Accounting",

account_invoice_tax_required/models/account_invoice.py

@@ -1,6 +1,7 @@
 # -*- coding: utf-8 -*-
 # Copyright 2015 - Camptocamp SA - Author Vincent Renaville
 # Copyright 2016 - Tecnativa - Angel Moya <odoo@tecnativa.com>
+# Copyright 2019 - Tecnativa - Pedro M. Baeza
 # License AGPL-3.0 or later (http://www.gnu.org/licenses/agpl).
 
 from odoo import models, api, _
@@ -26,7 +27,7 @@ class AccountInvoice(models.Model):
             )
 
     @api.multi
-    def invoice_validate(self):
+    def action_invoice_open(self):
         # Always test if it is required by context
         force_test = self.env.context.get('test_tax_required')
         skip_test = any((
@@ -40,4 +41,4 @@ class AccountInvoice(models.Model):
         ))
         if force_test or not skip_test:
             self._test_invoice_line_tax()
-        return super(AccountInvoice, self).invoice_validate()
+        return super(AccountInvoice, self).action_invoice_open()