mapbox-gl-directions.js: Object dosen´t support property or method 'assign on IE11

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Migrated from https://github.com/mapbox/mapbox-gl-js/issues/4316, originally by @Liverson

> Hello,
>
> on IE11 there is an error when i use mapbox-gl-directions.js to set waypoints on my map.
> On all other browsers incl. Edge the waypoints are shown fine.
> I tested with mapbox-gl-directions v3.0.3 and 3.1.1
>
> I got the following error message in the console:
> _Object dosen´t support property or method 'assign'_
> _mapbox-gl-directions.js (6518,7)_
>
> Breakpoint in mapbox-gl-directions.js:
> _this.originInput = new _geocoder2.default(Object.assign {} ...._
>
> When i comment out the following code, the page loading without errors.
> Seems map.addControl(directions) triggers the issue
>
> ```
> var directions = new MapboxDirections({
> accessToken: mapboxgl.accessToken,
> unit: 'metric',
> profile: 'cycling',
> controls: false,
> interactive: false
> });
>
> map.addControl(directions);
> ```
> Greets,
> Oliver

Start with mapbox-gl-directions.js around line 6518 and reproduce the failure by calling map.addControl with the directions configuration shown. Check how the Object.assign call is handled for IE11 compatibility. Done means adding the directions control no longer raises the reported Object.assign error in IE11 while preserving the existing behavior in other browsers.

Written by the indexing model from the issue text.