fix trailing comma for MapService providers

stop passing nested array to Tasks.Query.fields and assume that ObjectId field will already be present by virtue of our metadata check [here](https://github.com/Esri/esri-leaflet-geocoder/blob/master/src/Providers/MapService.js#L87).

modified existing tests to ensure no trailing comma is encountered in outFields parameter
made a few other tweaks to existing tests

fixes #46