Fix WP-Admin redirect loop after enabling SSL / HTTPS.

wp-config-snippet.php

<?php
/*
 * Source: http://wordpress.stackexchange.com/a/220896
 * Used behind a varnish / nginx reverse proxy setup.
 */
 
/** SSL */  
define('FORCE_SSL_ADMIN', true);  
// in some setups HTTP_X_FORWARDED_PROTO might contain  
// a comma-separated list e.g. http,https  
// so check for https existence  
if (strpos($_SERVER['HTTP_X_FORWARDED_PROTO'], 'https') !== false)  
    $_SERVER['HTTPS']='on';