Cursor displayed incorrectly in 'draw_polgon_mode'

Nobody has claimed this yet.

I've tested this behavior locally and through the Mapbox provided [example]((https://www.mapbox.com/mapbox-gl-js/example/mapbox-gl-draw/)). I'm assuming that the issue resides in the [code](https://github.com/mapbox/mapbox-gl-draw/blob/master/src/ui.js) that deals with updating the classes on the containing element for the map, as the correct class is not attached. This might mean this issue is broader than just one draw mode.

**mapbox-gl-js version**: 0.50.0
**mapbox-gl-draw version**: 1.0.9

### Steps to Trigger Behavior

1. Open the [Mapbox GL Draw example](https://www.mapbox.com/mapbox-gl-js/example/mapbox-gl-draw/) in Internet Explorer 11
2. Start drawing a polygon

### Expected Behavior
The cursor changes to a crosshair to indicate a polygon can be drawn on the map.

### Actual Behavior
The cursor sometimes changes to a crosshair, but inconsistently.

Start with src/ui.js and reproduce the behavior in the Mapbox GL Draw example using Internet Explorer 11, beginning a polygon draw. Inspect how classes are updated on the map’s containing element. Done means the polygon drawing mode consistently applies the crosshair cursor when drawing begins.

Written by the indexing model from the issue text.