Support endpoints where items are nested by ID

e.g. an endpoint might be the following shape:

```
GET /widgets?ids=1,2,3,6,8
```
_returns_
```json
{
	"1": {
		"widgetName": "foo"
	},
	"2": {
		"widgetName": "bar"
	},
	"5": {
		"widgetName": "baz"
	},
	"8": {
		"widgetName": "qux"
	}
}
```

We currently don't support this as a batch endpoint - we've seen this in the wild, and should support it :)