[MC-73678] Gamerule Typo's seems to make new gamerules

When doing the command /gamerule and misspelling mobSpawning or spelling it differently, It adds it to the list of gamerules.

Linked issues

Intended, see http://minecraft.gamepedia.com/Commands#gamerule.
However I admit that a way to remove self-defined gamerules would be useful.

EDIT: Whoops, didn't see that Sonic already posted (though it still says I posted before him for some reason).