Fix invalid request when Content-Length is unset.

"Invalid request (Malformed HTTP request)" and "Empty reply from server" is
returned when attempting to unset the Content-Length header by sending a
"Content-Length: " header.

src/Curl/Curl.php

@@ -442,7 +442,6 @@ class Curl
             $url = $this->baseUrl;
         }
         $this->setURL($url);
-        $this->unsetHeader('Content-Length');
         $this->setOpt(CURLOPT_CUSTOMREQUEST, 'PATCH');
         $this->setOpt(CURLOPT_POSTFIELDS, $this->buildPostData($data));
         return $this->exec();