Adds customer attaching test

tests/CreditCardTest.php

@@ -1,10 +1,28 @@
 <?php
 
 use CraigPaul\Moneris\Crypt;
+use CraigPaul\Moneris\Customer;
 use CraigPaul\Moneris\CreditCard;
 
 class CreditCardTest extends TestCase
 {
+    /**
+     * @var \CraigPaul\Moneris\CreditCard
+     */
+    protected $card;
+
+    /**
+     * Set up the test environment.
+     *
+     * @return void
+     */
+    public function setUp()
+    {
+        parent::setUp();
+
+        $this->card = CreditCard::create($this->visa, '2012');
+    }
+
     /** @test */
     public function it_can_instantiate_via_the_constructor()
     {
@@ -24,4 +42,14 @@ public function it_can_instantiate_via_a_static_create_method()
         $this->assertObjectHasAttribute('number', $card);
         $this->assertObjectHasAttribute('expiry', $card);
     }
+
+    /** @test */
+    public function it_can_attach_a_customer_to_a_credit_card()
+    {
+        $customer = Customer::create();
+
+        $this->card->attach($customer);
+
+        $this->assertEquals($customer, $this->card->customer);
+    }
 }