Fix getting the latest tweets in the user timeline

[nud]

Commit ea9041dfdff introduced an issue where GetFavourites() was called instead of GetUserTimeline() when latest_tweet_id is not set (so, in other words, the first time).

We also simplify calling GetFavourites() or GetUserTimeline() as the default value for the max_id parameter is None, so it is useless to special-case latest_tweet_id being None.