Ensure bonusNetherBlazeSpawnPercent applies only to Blazes in Nether

I have a plugin that is spawning PigZombies in the overworld, and this bit of code was changing them to blazes; I've made the code check for the world the entity is spawning in before processing the change.

Also, thanks for writing such well-organized and easy-to-troubleshoot code.  I hope you haven't been feeling a bunch of grief by my constant nagging; rather, I really enjoy this plugin and have been using it in conjunction with a couple others to make a very difficult server I am thoroughly enjoying.  If I ever get it ready for public viewing, I'll be sure to clue you in on it!  Much obliged for all your hard work!