Adding curl error code 18 to retry behavior
Fix #437
check exception class

I got this fatal error today : PHP Fatal error:  Call to undefined method Guzzle\Http\Exception\ServerErrorResponseException::getCurlHandle() in /home/frontoffice/instances/lekino-video_web/shared/vendor/guzzle/guzzle/src/Guzzle/Plugin/Backoff/BackoffPlugin.php on line 95
Only setting CURLOPT_NOBODY when the method of a request sends a body. Closes #472
Fixing cache plugin: Default revalidation handler should check the Cache-Control directive on the response, not the request.
Fixed typos