Stopped resetting of gamemode during login.

The way it was prior to this was basically always take the World's gamemode specified in worlds.yml and apply it to the player when they log in. This change makes it so if its null, apply the world's default gamemode but if they have one specified VanillaData.GAMEMODE (Creative, Survival, Adventure) then to use that instead.
Signed-off-by: Steven Downer <grinch@outlook.com>