added more accurate test for add_on_empty

with the current tests, if delete the assignment of is_empty in add_on_empty method
the tests not fail. With this test, if we delete is_empty, the test fails
Merge pull request #9324 from acapilleri/add_on_empty

added more accurate test for add_on_empty
Better not use tab chars in the generator template
Improve upgrading guide with ActionDispatch::BestStandardsSupport removal
Merge pull request #9331 from robertomiranda/upgrading-guide

Improve upgrading guide with ActionDispatch::BestStandardsSupport remova...