The letter 'z' was not a possible letter in mutation

[lazartsarev]

First of all thank you for the code that you have provided, it saved me a lot of time! I encountered this bug while messing around the java code. I tried to use the algorithm with a string that contained 'z' and iterating, because it was not possible for 'z' to occur in the mutation. I fixed this in the java code and in C#, clojure, python, ruby and scala.