Fixed BillingReports request method

when there is no report available for specified period (204 No content)

Author: pashamesh

src/BillingReports.php

@@ -111,6 +111,12 @@ public function get_id()
     public function save()
     {
         $header = parent::post(null, "BillingReport", $this->to_array());
+
+        if (!isset($header['Location']))
+        {
+            return $this;
+        }
+
         $splitted = explode("/", $header['Location']);
         $this->Id = end($splitted);