body: better outputs for object when debugging(development)

I found a little lame part of this module
Sometimes in my debugging workflow(development env var to be set), I wanna use browser to view my JSON better, in express, I can define that string by myself like `JSON.stringify(obj, null, 4)`, and in koajs, `res.body=` is a true awesome setter that would convert object to JSON like this:
```
{"foo":5,"bar":6}
```
But what we actually expected in debugging time is following in development:
```
{
  "foo": 5,
  "bar": 6
}
```

Then based on this, I think `response` would be more hummable in all browsers when `NODE_ENV` has been set to `development`, this is the easiest way.

/cc @koajs