Fix Array to string conversion warning in AltSessions.php

[austinginder]

Summary

Fixes #765

When a cookie value is an array and the expected type annotation is string, the (string)$value cast on line 227 triggers a PHP "Array to string conversion" warning on every POST /wp-json/cleantalk-antispam/v1/alt_sessions request.

Change

Added an is_array() check before the cast. Array values are JSON-encoded via wp_json_encode() instead of cast directly.

// Before
$cookies_array[$name] = (string)$value;

// After
$cookies_array[$name] = is_array($value) ? wp_json_encode($value) : (string)$value;