Fix content type for ImageMapLayer
Fix content type for ImageMapLayer

The default server response content type per the [docs](http://esri.github.io/esri-leaflet/api-reference/layers/image-map-layer.html#options) for an `imageMapLayer` should be `image`, not `json`.
RuXxar/esri-leaflet,Esri/esri-leaflet,Esri/esri-leaflet,RuXxar/esri-leaflet,RuXxar/esri-leaflet,Esri/esri-leaflet