charset is case-insensitive in Content-Type but we downcase here for …

…consistency with our other codebases;

at the same time, remove charset from Accept header as it isn't widely supported

lib/WebDriver/Service/CurlService.php

@@ -41,8 +41,8 @@ public function __construct($defaultOptions = array())
     public function execute($requestMethod, $url, $parameters = null, $extraOptions = array())
     {
         $customHeaders = array(
-            'Content-Type: application/json;charset=UTF-8',
-            'Accept: application/json;charset=UTF-8',
+            'Content-Type: application/json;charset=utf-8',
+            'Accept: application/json',
         );
 
         $curl = curl_init($url);