wpdb::escape was deprecated in Wordpress 3.6. However two calls were
still made to it by settings-debug.php. 'esc_sql'
(http://codex.wordpress.org/Function_Reference/esc_sql) is functionally
similar but with "stronger" escaping
(http://wordpress.org/support/topic/wordpress-36-rc2-last-minute-wpdbescape-deprecation-is-not-nice).