Replacement of fast-stable-stringify causes IE11 error

[sroettering]

Internet Explorer throws an error when I add layers to a map. The error comes from a call to Object.keys() which is implemented slightly different in IE. According to this Stackoverflow thread the IE version of Object.keys() throws a type error if a non-object is passed as an argument.

mapbox-gl-js version: 0.40.0

Steps to Trigger Behavior

1. Open IE11
2. Call the new stringify function in /src/style-spec/group_by_layout.js

Expected Behavior

No error thrown

Actual Behavior

Error thrown: Object.keys: argument is not an Object

[jfirebaugh]

Duplicate of #5312. This was fixed in 0.40.1.