HP-1631: created CustomerState

Author: VadymHrechukha

src/customer/Customer.php

@@ -89,7 +89,7 @@ public function getState(): ?string
 
     public function isDeleted(): bool
     {
-        return $this->state === 'deleted';
+        return CustomerState::isDeleted($this);
     }
 
     public static function fromArray(array $info)

src/customer/CustomerState.php

@@ -0,0 +1,13 @@
+<?php declare(strict_types=1);
+
+namespace hiqdev\php\billing\customer;
+
+enum CustomerState: string
+{
+    case DELETED = 'deleted';
+
+    public static function isDeleted(CustomerInterface $customer): bool
+    {
+        return self::tryFrom((string)$customer->getState()) === self::DELETED;
+    }
+}