[patch] Word count sample has a flawed addToMutationMap, fix

The WordCount example shows how to use client.batch_mutate, and has a helper method for building a mutation map. While the example works properly, the example addToMutationMap is flawed in that it won't allow adding of multiple columns to the same row, as is what is needed to perform a 'sql like insert' operation, which is the most likely example someone learning cassandra will want to do. Fixed the sample addToMutationMap code so that it works correctly for multi column inserts in one 'row'.