fetch

The documentation for mutate talks about local mutation for faster feedback, but the documented way to use it only mutates after a promise has resolved - in other words, not immediately.

As a result, I've found myself writing code in this kind of pattern:

mutate(path, { ...data, patch }, false); // mutate immediately, don't revalidate
await patchData(patch); // await the 

From time to time i see ppl using data instead of body which is wrong.
happened just recently in node-fetch/node-fetch#481 (comment)

how do you feel about adding a little warning message that can only appear once. (kinda like node reports that you are using experimental apis)

if ('data' in bodyInit && notWarned) {
  console.warn('use body i

For some reason Amazon REST api's require the content-type to be set to "application/x-amz-json-1.1" instead of the standard "application/json"

Currently the content-type header is set automatically for json data overriding any headers passed in to the request.

It would be useful to first check if the header is set before overriding it:

if (!this.request.headers.set('content-type')) {

Presumably these should result in a network error. The current specification doesn't state anything and it's not tested either.

See the README for instructions on where to add tests. In the specification the "blob" case of https://fetch.spec.whatwg.org/#scheme-fetch needs updating to handle current URL’s blob URL entry being null.