Support overriding "isModern" in featurelayer options

[spoilsportmotors]

• Version of Leaflet (L.version):
  1.0.3
• Version of esri Leaflet (L.esri.VERSION):
  2.0.7

Steps to reproduce the error:

We're running ArcGIS Server 10.4.1, which supports geojson output. Usually it's fine, but we get errors back occasionally of the form:

{"error":{"code":400,"message":"Failed to execute query.","details":[]}}
Server error logs merely say: "General geodatabase error occured."

The queries look pretty innocuous (not noticably different than those which succeed):
https://our_dev_server_name.us/arcgis/rest/services/some_service/MapServer/5/query?returnGeometry=true&outSr=4326&outFields=*&inSr=4326&geometry=%7B%22xmin%22%3A-117.1966552734375%2C%22ymin%22%3A34.05550388259356%2C%22xmax%22%3A-117.19596862792969%2C%22ymax%22%3A34.05607276338367%2C%22spatialReference%22%3A%7B%22wkid%22%3A4326%7D%7D&geometryType=esriGeometryEnvelope&spatialRel=esriSpatialRelIntersects&geometryPrecision=6&f=geojson

If I test this query in my browser, I get the same error; changing 'f=geojson' to 'f=json' allows the query to succeed.

As a work-around we've disabled the check for "isModern", and all layers draw as expected.

I'd be happy to submit a PR to add "isModern" to the options, so we can it at layer creation time, at least until we hear from the server team about the source of our geojson woes.

• Optional: I'm not using the CDN, I'm loading/bundling the library using webpack.

[jgravois]

thanks for the report.

I'd be happy to submit a PR to add "isModern" to the options, so we can it at layer creation time, at least until we hear from the server team about the source of our geojson woes.

please do!

[spoilsportmotors]

Can someone check if #935 is acceptable?