Don't stringify if there is not a body.

Example:
```js
request({ uri: 'some uri', json: true }, callback)
```

This will actually call `JSON.stringify` on `null` which results in the
string `'null'`.
Merge commit 'a313e3a7444a57a20670e118c2c5d141b48a06e4'
Handle options.qs query parameters
Merge branch 'qs'