Keep lines order when creating invoice lines

The itervalues introduced in changeset 7d28de2f84e4 doesn't keep the order of
the lines so it must loop over the lines to keep the order.

issue4794
review23431002

purchase.py

@@ -708,7 +708,9 @@ class Purchase(Workflow, ModelSQL, ModelView, TaxableMixin):
             return
 
         invoice = self._get_invoice_purchase(invoice_type)
-        invoice.lines = list(chain.from_iterable(invoice_lines.itervalues()))
+        invoice.lines = list(chain.from_iterable(
+                invoice_lines[l.id] for l in self.lines
+                if l.id in invoice_lines))
         invoice.save()
 
         Invoice.update_taxes([invoice])