Bug: `ResponseTest::testSetLastModifiedWithDateTimeObject` depends on time

[kenjis]

There was 1 failure:

1) CodeIgniter\HTTP\ResponseTest::testSetLastModifiedWithDateTimeObject
Failed asserting that two strings are identical.
--- Expected
+++ Actual
@@ @@
-'Sat, 11 Mar 2000 02:29:41 GMT'
+'Sat, 11 Mar 2000 02:29:40 GMT'

/home/runner/work/CodeIgniter4/CodeIgniter4/tests/system/HTTP/ResponseTest.php:248

https://github.com/codeigniter4/CodeIgniter4/runs/4389996488?check_suite_focus=true