REST API: Remove trailing slashes when preloading requests and add unit tests for it.

Props antonvlasenko.
Fixes #51636.

git-svn-id: https://develop.svn.wordpress.org/trunk@51648 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/rest-api.php

@@ -2818,6 +2818,11 @@ function rest_preload_api_request( $memo, $path ) {
 		}
 	}
 
+	$path = untrailingslashit( $path );
+	if ( empty( $path ) ) {
+		$path = '/';
+	}
+
 	$path_parts = parse_url( $path );
 	if ( false === $path_parts ) {
 		return $memo;