def authorize_from_existing(access_token, refresh_token, expires_at)
      @access_token = OAuth2::AccessToken.new(oauth_client, access_token, {
        refresh_token: refresh_token,
        expires_at:    expires_at
      })
    end
def authorize_from_existing(access_token, refresh_token, expires_at)
      @access_token = OAuth2::AccessToken.new(oauth_client, access_token, {
        refresh_token: refresh_token,
        expires_at:    expires_at
      })
    end
Initialize a new Gecko client object with an instantiated OAuth2::Client

 @param [String] client_id
 @param [String] client_secret
 @param [#to_hash] extra options hash to pass to the OAuth2::Client

 @return [undefined]

 @api private
 Authorize client from existing access and refresh token

 @example
   client.authorize_from_existing("ABC", "DEF", "1393982563")

 @param [String] access_token
 @param [String] refresh_token
 @param [String] expires_at

 @return [OAuth2::AccessToken]

 @api public