Adding this snippet to the functions.php of your wordpress theme will redirect visitors to the page they were viewing after logging in.

if ((isset($_GET['action']) && $_GET['action'] != 'logout') ||
(isset($_POST['login_location']) &&
!empty($_POST['login_location'])) ) {
       add_filter('login_redirect', 'my_login_redirect', 10, 3);
       function my_login_redirect() {
                $location = $_SERVER['HTTP_REFERER'];
                wp_safe_redirect($location);
                exit();
        }
}