public function testUpcoming()
 {
     authorizeFromEnv();
     $customer = self::createTestCustomer();
     Conekta_InvoiceItem::create(array('customer' => $customer->id, 'amount' => 0, 'currency' => 'usd'));
     $invoice = Conekta_Invoice::upcoming(array('customer' => $customer->id));
     $this->assertEqual($invoice->customer, $customer->id);
     $this->assertEqual($invoice->attempted, false);
 }