support non-json request bodies

[bobzoller]

I'd like to use this with the bill.com API, but their API accepts form-encoded request bodies and returns json responses. This bit of code prevents me from providing the body as a form-encoded string:

json-fetch/src/get_request_options/index.js

Lines 10 to 13 in 5457157

	if (jsonFetchOptions.body !== undefined) {
	parsedOptions.body = JSON.stringify(jsonFetchOptions.body);
	parsedOptions.headers['Content-Type'] = 'application/json';
	}

[bobzoller]

seems like there's three options:
a. json-fetch doesn't care about your half-json API; use a different library
b. only JSON.stringify if body is an object (breaking change, but I would argue a standard pattern in other request libraries)
c. add rawBody or some other parameter