Check if header exists before forcing it

Author: rjzondervan

src/Service/RequestService.php

@@ -817,7 +817,7 @@ public function proxyHandler(array $data, array $configuration, ?Source $proxy =
                 unset($headers['Content-Length']);
             }
 
-            if ($data['accept'] !== $headers['content-type'][0]) {
+            if (isset($headers['content-type']) === false || $data['accept'] !== $headers['content-type'][0]) {
                 $headers['content-type'] = $data['accept'];
             }