JSON return event.fields structure is an array should be an hash

GET /group/:gid/events used to return

```
{
    ... event attr ...,
    fields : {
        key1 : value1,
        key2 : value2
    }
}
```

and now after changes for issue #105 returns

```
{
    ... event attr ...,
    fields : [
        { id : key1, value : value1 },
        { id : key2, value : value2 }
    ]
}
```
