Added string array support to ESJsonEncoder

This is a quick tweak to allow ESJsonEncoder to handle multiple string values.
I appreciate your clever trick here which allows you to have a single array processing function for all of the types. I'm not to keen on the use of reflect, though. Go's reflection is notoriously slow, and this manual string JSON generation is already slow enough.

Plus, you already *know* the type of field you're dealing with, b/c the only place this function is called is from inside the code below where we're switching on message type. Why don't you just pass the type in to the array function so you don't have to duplicate your efforts using reflection?