code: public function post(Array $postVars, Array $headers, $url=null)
      {
          try {
              $this->guzzleResponse = $this->_client->request(
                  'POST',
                  $url,
                  array('multipart' => $postVars, 'headers' => $headers)
              );
          } catch (RequestException $e) {
              throw new GreenhouseAPIResponseException($e->getMessage(), 0, $e);
          }
          
          return (string) $this->guzzleResponse->getBody();
      }
docs: Post to the application endpoint.
  
  @params  Array   $postVars       A guzzle compliant multipart array of post parameters.
  @params  Array   $headers        This should only contain the Authorization header.
  @params  string  $url            This can be left blank.  Url is set in the constructor.
  @return  string
  @throws  GreenhouseAPIResponseException  for non-200 responses