this test should actually check if retuning a new value will replace the keys.

I would recommend something like:

```js
var compare = {
   'foo-0': 0,
   'bar-1': 1,
   'baz-2': 2 
};

var thisObj = {};
var newObj = mapKeys(obj, function(key, val, o){
   // make sure it receives all arugments
   expect(o).toBe(obj);
   expect(this).toBe(thisObj);

   return key +'-'+ val;
}, thisObj);

expect(newObj).toEqual(compare);
```