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

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

I would refer to the set method by its full name, `Map.prototype.set`, as currently this table uses the convention that prop names in quotes are instance own properties (compare `Function "name" property`). And instead of "Map object", I'd use `this`, to explicate that the original object is returned.