do not add leading slash when there is one already fix #5473 (#5474)

Author: nicholascus

src/Codeception/Module/REST.php

@@ -577,7 +577,7 @@ protected function execute($method, $url, $parameters = [], $files = [])
         // allow full url to be requested
         if (strpos($url, '://') === false) {
             $url = $this->config['url'] . $url;
-            if ($this->config['url'] && strpos($url, '://') === false && $this->config['url'][0] !== '/') {
+            if ($this->config['url'] && strpos($url, '://') === false && strpos($url, '/') !== 0 && $this->config['url'][0] !== '/') {
                 $url = '/' . $url;
             }
         }