fix content type

src/MirfalahTransport.php

                 'apiKey' => $this->apiKey,
                 'signature' => hash_hmac('sha1', $body, $this->secret),
             ],
+            RequestOptions::HEADERS => [
+                'Content-Type' => 'text/plain; charset=utf-8',
+            ],
             RequestOptions::BODY => $body,
         ]);
     }