[11.x] Send invoice (#942)

* Use sendInvoice() when collection method is 'send_invoice'

In some edge cases (like mine) we have clients that can't use a credit card, but we still want to have all the data in Stripe. So we create a subscription with collection_method = 'send_invoice' and the manually sent the invoices to be paid, once we received payment we just change the status manually in Stripe.

´´´
$model->invoice(['collection_method' => 'send_invoice']);
$subscription->invoice(['collection_method' => 'send_invoice']);
´´´

When you use this collection method you can't apply $stripeInvoice->pay() function, you need to use the $stripeInvoice->sendInvoice();

When using the $stripeInvoice->pay() on when a colletion_method is 'send_invoice' it throws PaymentFailure, it doesn't require a payment method.

The only two options in 'collection_methods' is charge_automatically (default) or 'send_invoice' there fore using a simple if else statement.

https://stripe.com/docs/api/invoices/create#create_invoice-collection_method

* Use constant

Co-authored-by: Erik Larsson <eriklarsson@me.com>

src/Concerns/ManagesInvoices.php

@@ -76,7 +76,11 @@ public function invoice(array $options = [])
             /** @var \Stripe\Invoice $invoice */
             $stripeInvoice = StripeInvoice::create($parameters, $this->stripeOptions());
 
-            $stripeInvoice = $stripeInvoice->pay();
+            if ($stripeInvoice->collection_method === StripeInvoice::COLLECTION_METHOD_CHARGE_AUTOMATICALLY) {
+                $stripeInvoice = $stripeInvoice->pay();
+            } else {
+                $stripeInvoice = $stripeInvoice->sendInvoice();
+            }
 
             return new Invoice($this, $stripeInvoice);
         } catch (StripeInvalidRequestException $exception) {