Fixes check for -0 in Map.set polyfill

[jonbretman]

If an object with a null prototype is provided as a key to the Map polyfill then a TypeError is raised. I think this is expected:

$ node
> var key = Object.create(null);
undefined
> 1/key;
TypeError: Cannot convert object to primitive value

This code was changed here and I think this is where the bug was introduced as I've tried the previous code (key === -0) and it seems to work fine.

[jonbretman]

@JakeChampion thanks for merging 👍 Is there any way to know when this is available on https://polyfill.io/v3/polyfill.js?

[JakeChampion]

I've added a test for this --> #133
I can let you know when it goes out

[jonbretman]

Thanks!

[JakeChampion]

The work to add this to polyfill-service is in this PR --> polyfillpolyfill/polyfill-service#1892