[FIX] account: fixed grouping of account.invoice.tax creation.

in details:
[FIX] added analytic account in grouping key of because we need to create different journal items for each of the lines having a different analytic account.
[REF] added a function on account.tax that returns the grouping key and that could be inherited in another (community) module

addons/account/models/account.py

@@ -602,6 +602,11 @@ class AccountTax(models.Model):
         if self.price_include:
             self.include_base_amount = True
 
+    def get_grouping_key(self, invoice_tax_val):
+        """ Returns a string that will be used to group account.invoice.tax sharing the same properties"""
+        self.ensure_one()
+        return str(invoice_tax_val['tax_id']) + '-' + str(invoice_tax_val['account_id']) + '-' + str(invoice_tax_val['account_analytic_id'])
+
     def _compute_amount(self, base_amount, price_unit, quantity=1.0, product=None, partner=None):
         """ Returns the amount of a single tax. base_amount is the actual amount on which the tax is applied, which is
             price_unit * quantity eventually affected by previous taxes (if tax is include_base_amount XOR price_include)

addons/account/models/account_invoice.py

@@ -567,7 +567,8 @@ class AccountInvoice(models.Model):
                 if not val.get('account_analytic_id') and line.account_analytic_id and val['account_id'] == line.account_id.id:
                     val['account_analytic_id'] = line.account_analytic_id.id
 
-                key = str(tax['id']) + '-' + str(val['account_id'])
+                key = self.env['account.tax'].browse(tax['id']).get_grouping_key(val)
+
                 if key not in tax_grouped:
                     tax_grouped[key] = val
                 else: