Elasticsearch json fields fixes

Allows the "fields" parameter to work in ESLogstashV0Json (previously not functional)

Corrects the fields parameter in ESJsonEncoder if "Fields" is the first listed field (comma separates correctly)

TODO: is it possible to set the @type to match the _type set in coordinates.bro?
I'm not a Go dev, and I couldn't work out how to export the function "interpolateFlag"