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

Look at the existing groupBy filter implementation in the angular-filter source code, likely in a file like angular-filter.js or a filters directory. The test shows the desired behavior: groupBy should accept an array of keys and group objects by the combined values of those keys, forming a composite key string like '1,1'. Start by finding where groupBy is defined, then modify it to handle an array of keys. Run the provided test to verify the change works.

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