extra tests for `Map`, see data-es6.js

Sorry, the compatibility data i have is only for Chrome, Firefox, IE 11.

map.get(+0) == map.get(-0); // Chrome 38 - OK
but with `forEach` you can see, that it is stored as `-0`

Later the spec was changed for this case:
see https://people.mozilla.org/~jorendorff/es6-draft.html#sec-map.prototype.set : `Let p be the Record {[[key]]: key, [[value]]: value}.`