[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.

Environment

windows server 2012, Java 7 version 67
Intel Core i5-2500k CPU @ 3.3GHz
Ram: 8192MB
-ram custom to minecraft 512MB Xms 5GXmx

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).