code: func (a clientOAuthAuthentication) authenticate(req *http.Request, endpoint string, params []byte) error {
  	// Ensure tokens havent expired
  	if time.Now().UTC().Unix() > a.Tokens.ExpireTime {
  		return errors.New("The OAuth tokens are expired. Use refreshTokens to refresh them")
  	}
  	req.Header.Set("Authorization", "Bearer "+a.Tokens.AccessToken)
  	return nil
  }
docs: // Client OAuth authentication requires us to attach an unexpired OAuth token to
  // the request header