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')) {

processRequestBodyChunk or even processRequestBodyChunkSize would be more representative and either of those would make it clear it's different from processResponseBody, which is only invoked once and is actually passed a response.

Thanks to Tab for making me realize this.

Marking this as a good first issue as it is somewhat straightforward to rename this in various places, but note that thi