Default world difficulty and game rule settings do not update when switching world type to Debug Mode

Default world difficulty and game rule settings do not update to the default settings of debug worlds when switching to that world type.

Steps to reproduce:

1. Go to the world creation menu. Do not change the difficulty nor the game rules.
2. Go to 'More World Options...' and switch the world type to Debug Mode.
3. Click 'Done.' Take note that the difficulty is set to Normal and the game rule doDaylightCycle is on.
4. Generate the world and check the difficulty as well as the aforementioned game rule.
   •  The difficulty is set to Peaceful
   •  doDaylightCycle is set to false.

Recreating a debug world sets the difficulty and doDaylightCycle to their intended debug world defaults.