Use site_url for route-based RedirectResponse

base_url does not actually add the base path if the given URL is
absolute (i.e., starts with a slash) and does not add the indexPage.
Both is necessary for the redirect response to work correctly in case
of routes. site_url handles this correctly.

Fixes codeigniter4#2119

system/HTTP/RedirectResponse.php

@@ -91,7 +91,7 @@ public function route(string $route, array $params = [], int $code = 302, string
 			throw HTTPException::forInvalidRedirectRoute($route);
 		}
 
-		return $this->redirect(base_url($route), $method, $code);
+		return $this->redirect(site_url($route), $method, $code);
 	}
 
 	/**