Fixed issue where new GKLeaderboard object's category property was being set to the saved GKLeaderboard object.

Fixing this seemed to cause the object to be removed from the
_leaderboards ivar, so I had to add the check to prevent an
NSRangeException. Please feel free to clean up my solution if you can
discover why this is occurring.
Merge pull request #4 from michaelpatzer/master

Fixed issue where GKLeaderboard object was trying to be encoded.