polyfill.Fetch object request body (application/json ) being converted to string

[mikemonteith]

It seems that fetch() calls with a JSON body, the JSON is being converted to a string.

This is the whatwg-fetch way of doing a JSON post:

fetch(URL, {
  method: 'POST',
  headers: {
    'Content-Type': 'application/json',
  },
  body: JSON.stringify({abc: 123}),
});

I have also tried

fetch(URL, {
  method: 'POST',
  headers: {
    'Content-Type': 'application/json',
  },
  body: {abc: 123},
});

I think that polyfill.Fetch is converting my JSON-string into a string-containing-a-JSON-string.

[wkh237]

@mikemonteith , FYI, I've published 0.10.0-beta.4 which includes your PR 👍