LTI lti_verify_with_keyset fails behind a proxy

the function lti_verify_with_keyset in mod/lti/locallib.php fails because it uses php function file_get_contents to retrieve url

if I use curl object instead of file_get_contents the url resource can be retrieved with the proxy settings

this happens only if the keyset is not already cached

        // CURL VERSION         

        $curl = new curl(); 

        $keyset = $curl->get($keyseturl); 

        // ORIGINAL VERSION

        // $keyset = file_get_contents($keyseturl);