def sign_out_url(redirect_url=nil)
      auth_server_url = Addressable::URI.parse(endpoint)
      auth_server_url.path = '/users/sign_out'
      auth_server_url.query_values = {redirect_url: redirect_url} if redirect_url
      auth_server_url.to_s
    end
def sign_out_url(redirect_url=nil)
      auth_server_url = Addressable::URI.parse(endpoint)
      auth_server_url.path = '/users/sign_out'
      auth_server_url.query_values = {redirect_url: redirect_url} if redirect_url
      auth_server_url.to_s
    end
Return the URL for signing out of the auth server.
 Clients should redirect to this URL to globally sign out.

 @param [String] redirect_url the URL that the auth server should redirect back to after sign out
 @return [String] the auth server endpoint for signing out