groupBy multiple key

```
it('should allow groupBy multiple key', function() {
var arrays = [
{ key1: 1, key2: 1, data: {} },
{ key1: 1, key2: 1, data: {} },
{ key1: 1, key2: 2, data: {} },
{ key1: 1, key2: 2, data: {} },
{ key1: 2, key2: 3, data: {} },
{ key1: 2, key2: 3, data: {} },
{ key1: 2, key2: 4, data: {} },
{ key1: 2, key2: 4, data: {} }
];

expect(filter(arrays, ['key1', 'key2'])).toEqual({
'1,1': [arrays[0], arrays[1]],
'1,2': [arrays[2], arrays[3]],
'2,3': [arrays[4], arrays[5]],
'2,4': [arrays[6], arrays[7]]
});
});
```

should allow to group by multiple key instead of only 1 key