Support request style options.json

This will make writing tests easier and get close to the `request` interface

```js
MockRequest({
  method: 'POST',
  url: '/foo',
  json: { 'status': 42 }
})
```

instead of 

```js
var req = MockRequest({
  method: 'POST',
  url: '/foo'
})

...

req.end(JSON.stringify({ status: '42' }))
```