OAuth2AccessToken->getAccessTokenExpiresAt has wrong return data type

[gmariani]

/**
 * Return the expiration date of Access Token
 * @return Date
 */
public function getAccessTokenExpiresAt(){
    if(!empty($this->accessTokenExpiresAt))
    {
      return $this->getDateFromSeconds($this->accessTokenExpiresAt);
    }else{
      throw new SdkException("The Expiration Time for OAuth 2 Access Token is not set.");
    }
}

It should be @return string, because getDateFromSeconds() just does :

/**
 * A helper function to convert Seconds to date
 * @param  int  $second
 * @return string
 */
private function getDateFromSeconds($seconds){
  return date('Y/m/d H:i:s', $seconds);
}