Give a default value to the param $post_string in http()

This private function is called with and without this param.

src/Subscriber.php

@@ -181,7 +181,7 @@ private function change_subscription($mode, $topic_url, $http_function = false)
      *
      * @return mixed
      */
-    private function http($url, $post_string)
+    private function http($url, $post_string = false)
     {
 
         // add any additional curl options here