Use Map() instead of Object()

[ijlyttle]

Fix #446

I think using a Map() here will take care of the issue.

[JustinHaight]

Is there any way to give this a review? @tschaub

[tschaub]

Thanks for the proposed change, @ijlyttle, and for the prompt, @JustinHaight.

In #452, I updated the same function to use a Set, so I'll close this.

The change is included in the v5.0.0 release.