Check rule only if _check_access is set and enforce companies rule

issue4080
review343891009

tests/test_account_dunning_letter.py

@@ -6,10 +6,10 @@ import trytond.tests.test_tryton
 from trytond.tests.test_tryton import ModuleTestCase
 from trytond.tests.test_tryton import doctest_teardown, doctest_checker
 
-from trytond.modules.company.tests import CompanyMultiValueTestMixin
+from trytond.modules.company.tests import CompanyTestMixin
 
 
-class AccountDunningLetterTestCase(CompanyMultiValueTestMixin, ModuleTestCase):
+class AccountDunningLetterTestCase(CompanyTestMixin, ModuleTestCase):
     'Test AccountDunningLetter module'
     module = 'account_dunning_letter'