Fix: percentage field is now required when a tax type is percentage.

tax.py

@@ -211,6 +211,7 @@ class Tax(OSV):
     percentage = fields.Numeric('Percentage', digits=(16, 8),
             states={
                 'invisible': "type != 'percentage'",
+                'required': "type == 'percentage'",
             }, help='In %')
     type = fields.Selection([
         ('percentage', 'Percentage'),