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
Fix time_zone_options_for_select to not mutate TimeZones array

Previous implementation of time_zone_options_for_select did not dup the
ActiveSupport::TimeZone.all array.  When :priority_zones were provided
the method would reject! the zones from the memoized TimeZones array
thus affecting future requests to the server.  Essentially whatever
zones were specified as :priority_zones would show up for the first
request but then disappear from the time zone options on future
requests.
Improve upgrading guide with ActionDispatch::BestStandardsSupport removal
Merge pull request #9331 from robertomiranda/upgrading-guide

Improve upgrading guide with ActionDispatch::BestStandardsSupport remova...
Merge pull request #9330 from bdmac/time_zone_select_dup

Fix time_zone_options_for_select to not mutate TimeZones array
descriptive error message when AR adapter was not found. Closes #7313.
Merge pull request #8060 from senny/7313_descriptive_error_message_for_missing_adapter

descriptive error message when AR adapter was not found. Closes #7313
Whitespace :heart: and hash syntax change
Remove XML Parser from ActionDispatch

If you want an ability to parse XML parameters, please install
`actionpack-xml_parser` gem.