refactor: replace deprecated getHeader() with header()

Author: kenjis

user_guide_src/source/libraries/curlrequest/006.php

@@ -2,5 +2,5 @@
 
 echo $response->getStatusCode();
 echo $response->getBody();
-echo $response->getHeader('Content-Type');
+echo $response->header('Content-Type');
 $language = $response->negotiateLanguage(['en', 'fr']);

user_guide_src/source/libraries/curlrequest/012.php

@@ -1,5 +1,5 @@
 <?php
 
-if (strpos($response->getHeader('content-type'), 'application/json') !== false) {
+if (strpos($response->header('content-type'), 'application/json') !== false) {
     $body = json_decode($body);
 }