docs: fix cache driver example

Current cache storage example is not working, because it assigns data to non existent variable. I have also added cookie persistence, because sessions only lasts for 20minutes. In most cases we would like to maintain cart items for at least a week.

Author: AidasK

README.md

@@ -986,43 +986,53 @@ class DBStorage {
 }
 ```
 
-For example you can also leverage Laravel's Caching (redis, memcached, file, dynamo, etc) using the below example,
+For example you can also leverage Laravel's Caching (redis, memcached, file, dynamo, etc) using the example below. Exmaple also includes cookie persistance, so that cart would be still available for 30 days. Sessions by default persists only 20 minutes. 
 
 ```
-use Illuminate\Support\Facades\Cache;
+<?php
 
-class CacheStorage {
+namespace App\Cart;
 
-public function has($key)
-{
-    return Cache::get($key);
-}
+use Carbon\Carbon;
+use Cookie;
+use Darryldecode\Cart\CartCollection;
 
-public function get($key)
+class CacheStorage
 {
-    if($this->has($key))
+    private $data = [];
+    private $cart_id;
+
+    public function __construct()
     {
-        return new CartCollection(Cache::get($key));
+        $this->cart_id = \Cookie::get('cart');
+        if ($this->cart_id) {
+            $this->data = \Cache::get('cart_' . $this->cart_id, []);
+        } else {
+            $this->cart_id = uniqid();
+        }
     }
-    else
+
+    public function has($key)
     {
-        return [];
+        return isset($this->data[$key]);
     }
-}
 
-public function put($key, $value)
-  {
-    if($row = Cache::get($key))
+    public function get($key)
     {
-        // update
-        $row->cart_data = $value;
-        Cache::put($key, $value, now()->addMinutes(3600));
+        return new CartCollection($this->data[$key] ?? []);
     }
-    else
+
+    public function put($key, $value)
     {
-        Cache::put($key, $value, now()->addMinutes(3600));
+        $this->data[$key] = $value;
+        \Cache::put('cart_' . $this->cart_id, $this->data, Carbon::now()->addDays(30));
+
+        if (!Cookie::hasQueued('cart')) {
+            Cookie::queue(
+                Cookie::make('cart', $this->cart_id, 60 * 24 * 30)
+            );
+        }
     }
-  }
 }
 ```