prevent warning on access to undefined variable

Author: oscarotero

src/Http/CurlDispatcher.php

@@ -129,7 +129,7 @@ protected function exec(Url $url, array $options)
         });
 
         $curl->onBody(function ($string, stdClass $data) {
-            return !$data->isBinary;
+            return empty($data->isBinary);
         });
 
         curl_exec($connection);