Fix regression on `assert_redirected_to`.

`assert_redirected_to` would fail if there is no controller set on
a `ActionDispatch::IntegrationTest`, as _compute_redirect_to_location
would be called on the controller to build the url.

This regression was introduced after 1dacfbabf3bb1e0a9057dd2a016b1804e7fa38c0.

[fixes #14691]
Merge pull request #15084 from arthurnn/fix_assert_redirected_to

Fix regression on `assert_redirected_to`.