I was not able to get `NaiveBayes.predict()`'s return type to work here, even after using `.rdd()` and `toJavaRDD()` and even `new JavaRDD(...)`. The generic type `Double` always seemed to be lost, due to `ClassTag` voodoo I imagine. Hence this way. Maybe there was a way to make it work, but this works fine for the tiny example.