public function getAccessToken($code)
{
$postKey = version_compare(ClientInterface::VERSION, '6') === 1 ? 'form_params' : 'body';
$response = $this->getHttpClient()->post($this->getTokenUrl(), ['headers' => ['Accept' => 'application/json'], $postKey => $this->getTokenFields($code)]);
return $this->parseAccessToken($response->getBody());
}