Add current_period_start to getObjectParam for subscriptions

CRM/Stripe/Api.php

@@ -194,6 +194,9 @@ class CRM_Stripe_Api {
           case 'next_sched_contribution_date':
             return self::formatDate($stripeObject->current_period_end);
 
+          case 'current_period_start':
+            return self::formatDate($stripeObject->current_period_start);
+
           case 'cycle_day':
             return date("d", $stripeObject->billing_cycle_anchor);