fix test error for curl download

Author: inhere

src/Curl/CurlClient.php

@@ -334,11 +334,11 @@ public function request(string $url, $data = null, string $method = 'GET', array
 
         // if \CURLOPT_HEADER is FALSE, only return body. no headers data
         if (false === $this->getCurlOption(CURLOPT_HEADER, false)) {
-            $this->responseBody = $response;
+            $this->responseBody = (string)$response;
             $this->setResponseParsed(true);
         } else {
             // if CURLOPT_HEADER is TRUE, The raw response data contains headers and body
-            $this->rawResponse = $response;
+            $this->rawResponse = (string)$response;
             $this->parseResponse(); // parse raw response data
         }