Add specs to Iugu::Customer.fetch

Author: Lucas

spec/cassettes/Iugu_Customer/_fetch/should_return_the_customer.yml

@@ -67,7 +67,7 @@
       expect(customers.total).to eq(15)
     end
 
-    xit 'should return the customer', :vcr do
+    it 'should return the customer', :vcr do
       customer = Iugu::Customer.fetch(id: '2D3309067D0047B7AD55F38EBA0BA406')
 
       expect(customer.email).to eq('[email protected]')