Extend number length for american express (#30)

* Extend number length for american express

American Express card can also be 17 digits long, please test with these card details:

Card Number: 3455555555555504
CV2/CVV: 1234

* added 16 digits long amex card to tests

Author: stefanheimann

src/Cards/AmericanExpress.php

@@ -39,7 +39,7 @@ class AmericanExpress extends Card implements CreditCard
      *
      * @var array
      */
-    protected $number_length = [15];
+    protected $number_length = [15, 16];
 
     /**
      * CVC code length's.

tests/Unit/Cards/AmericanExpressTest.php

@@ -115,6 +115,7 @@ public function validNumbers(): Collection
             '344566883297843',
             '379811393604866',
             '378167408666270',
+            '3455555555555504',
         ]);
     }