Date/Time: Use strict comparison in is_new_day(), add a unit test.

Props pbearne.
Fixes #46627.

git-svn-id: https://develop.svn.wordpress.org/trunk@45375 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/functions.php

@@ -797,7 +797,8 @@ function wp_get_http_headers( $url, $deprecated = false ) {
  */
 function is_new_day() {
 	global $currentday, $previousday;
-	if ( $currentday != $previousday ) {
+
+	if ( $currentday !== $previousday ) {
 		return 1;
 	} else {
 		return 0;