Add invoice_date to API

CRM/Stripe/Api.php

@@ -115,6 +115,15 @@ class CRM_Stripe_Api {
             }
             return self::formatDate($timestamp);
 
+          case 'invoice_date':
+            if (!empty($stripeObject->status_transitions->finalized_at)) {
+              $timestamp = $stripeObject->status_transitions->finalized_at;
+            }
+            else {
+              $timestamp = $stripeObject->created;
+            }
+            return self::formatDate($timestamp);
+
           case 'subscription_id':
             return (string) $stripeObject->subscription;