[FIX] account: fixed group_lines method of invoices.

The group_lines method didn't make the sum of the quantity field, hence resulting in incorrect results when making product based statistics from the account.move.line records.

Courtesy of Luc De Meyer. Was PR #10551

addons/account/account_invoice.py

@@ -781,6 +781,9 @@ class account_invoice(models.Model):
                     line2[tmp]['tax_amount'] += l['tax_amount']
                     line2[tmp]['amount_currency'] += l['amount_currency']
                     line2[tmp]['analytic_lines'] += l['analytic_lines']
+                    qty = l.get('quantity')
+                    if qty:
+                        line2[tmp]['quantity'] = line2[tmp].get('quantity', 0.0) + qty
                 else:
                     line2[tmp] = l
             line = []