documentcloud/underscore — pull request #1024 by playerace (2013-03-17 05:25:47+00:00)
Title: Make groupBy preserve keys when grouping objects.

I've had this scenario when grouping objects such as
```javascript
{
1363402200: { "values": "1" },
1363402201: { "values": "5" },
1363402202: { "values": "17" }
}
```
and using groupBy doesn't preserve the keys as this becomes
```javascript
_.groupBy(obj, function(v, k) { return k % 2; })

{
"0": [
{ "values": "1" },
{ "values": "17" }
],
"1": [
{ "values": "5" }
]
}
```
And the one I prefer is this which preserves the keys as I need to do additional operations on it.
```javascript
{
"0": {
"1363402200": { "values": "1" },
"1363402202": { "values": "17" }
},
"1": {
"1363402201": { "values": "5" }
}
}
```

I'm not sure if this is a common use case as I've made "preserve keys" default instead of optional.