Simple mu-plugin (move to wp-content/mu-plugins/cloudflare-flexible-ssl-redirect-loop-fix.php) to fix cloudflare redirect and optionally force https

cloudflare-flexible-ssl-redirect-loop-fix.php

<?php

// Fix https detection
if( (isset($_SERVER['HTTP_CF_VISITOR']) && (strpos($_SERVER['HTTP_CF_VISITOR'], 'https') !== false)) || (isset($_SERVER['HTTP_X_FORWARDED_PROTO']) && (strpos($_SERVER['HTTP_X_FORWARDED_PROTO'], 'https') !== false)) ){
    $_SERVER['HTTPS'] = 'on';
}

// Also force https (optional)
if(empty($_SERVER['HTTPS']) || $_SERVER['HTTPS'] == "off"){
    $redirect = 'https://' . $_SERVER['HTTP_HOST'] . $_SERVER['REQUEST_URI'];
    header('HTTP/1.1 301 Moved Permanently');
    header('Location: ' . $redirect);
    exit();
}