Made json and multipart options coexist

Simply made the behavior of combining the options more consistent. For example I can have `json` option and `formData` option coexist, in other words getting the JSON parsed body in the callback and stuff like that.

Currently that's not possible for the `multipart` option, and if the only reason for this if/else https://github.com/mikeal/request/blob/master/request.js#L577-L582 was to make the header check inside the multipart method works https://github.com/mikeal/request/blob/master/request.js#L1420-L1425 I'm fixing that