Fix #10842. `assert_redirected_to` does not show user-supplied message.

Fix #10842

Issue: when `assert_redirected_to` fails due to the response redirect not
matching the expected redirect the user-supplied message (second parameter)
is not shown. This message is only shown if the response is not a redirect.

If ...

    assert_redirected_to login_path, "Expected redirect to login page"

... there is redirection not to `login_path` then we don't see "Expected redirect to login page." but only default "Expected response to be a redirect to /login but was a redirect to /other." message.


The issue was [first mentioned in 2011](https://rails.lighthouseapp.com/projects/8994/tickets/2477-assert_redirected_to-should-surface-the-message-in-all-fail-cases) but was not fixed in 3.* branch. I faced with it several days ago.

In 4.* `action_dispatch` tests helpers were refactored so 4.* has no such bug.