FormData with jsonPlus

[Kestutis]

Right now I can not pass FormData if I'm using jsonPlus middleware. All my responses comes as json, so I need to pass Accept header as json as well as parse response.
If I have FormData(want to upload file), I don't need Content-Type: application/json header. fetch-plus-json doesn't handle this case.

Workaround I'm using:

client.addMiddleware((request) => {
    if (request.options.body instanceof FormData) {
        request.options.headers["Accept"] = "application/json";
        return plusJson.handler();
    } else {
        return plusJson()(request);
    }
});