rest client.Logout should clear c.auth

[TheMeier]

c.Login returns if c.auth is not nil
https://github.com/RocketChat/Rocket.Chat.Go.SDK/blob/master/rest/users.go#L82

So in Logout() c.auth should be cleared. Of cause it would be also nice if it was cleared on other places in the code where it can be detected that the login of the client is not valid anymore