Update Cart.php

Fix MongoDB Driver issue Crinsane#478

src/Cart.php

@@ -389,11 +389,11 @@ public function restore($identifier)
             ->where('instance', $this->currentInstance())
             ->where('identifier', $identifier)->first();
 
-        $storedContent = unserialize($stored->content);
+        $storedContent = unserialize(data_get($stored, 'content'));
 
         $currentInstance = $this->currentInstance();
 
-        $this->instance($stored->instance);
+        $this->instance(data_get($stored, 'instance'));
 
         $content = $this->getContent();