Users with spaces in their name can join (offline) servers

Now I know offline-mode servers are generally frowned upon, and I don't know if issues relating to them can be created, but this is a bug nonetheless.

Since there's no username check for offline-mode servers, anyone with any name (even ones with spaces) are allowed to join the server, breaking every command that only has one arg for the username (e.g. /ban Bob 1 won't ban Bob 1 but instead only tries to ban Bob). This relates to MC-173354 even though it was closed due to the issue I'm describing right here. 

I know these kinds of servers are sorta left to defend for themselves, even though in (some) cases, you may need one (such as blocked on school wifi), and in that case you kinda want to prevent this 😛