Fix the inability to override commands

[Yexeed]

Currently, it is impossible to unregister a command using this method:

public static boolean unregisterCommand(Command command){
    command.setLabel(command.getName() + "__unregistered");
    return command.unregister(Server.getInstance().getCommandMap());
}

command.unregister() returns false because Command.allowChangesFrom() returns false. As old code shows, a commandMap passed as parameter should NOT be null (which is passes fine), and it should NOT be equal to command's known command map (this.commandMap), this sounds as complete nonsense because command SHOULD allow changes from a commandMap that is equal to this.commandMap and vice-versa. (Vanilla commands are all registered using same SimpleCommandMap instance so it should always return true when comparing this.commandMap to Server.getInstance().getCommandMap(), right?

P.S.
Also, this should prove my point.

• https://github.com/pmmp/PocketMine-MP/blob/stable/src/command/Command.php#L176

• (referred issue: unregister commands not working #2145)