Make sure all fetch options are passed (#46)

* make sure all fetch options are passed

* updating documentation

Author: santino

README.md

@@ -36,6 +36,11 @@ function fetchQuery(operation, variables) {
 
 The `credentials` param is passed to [`fetch`](https://developer.mozilla.org/en-US/docs/Web/API/WindowOrWorkerGlobalScope/fetch#Parameters). For XHR requests, [`XMLHttpRequest.withCredentials`](https://developer.mozilla.org/en-US/docs/Web/API/XMLHttpRequest/withCredentials) is mapped to true when `credentials: 'include'`.
 
+#### Handling additional request settings
+
+When `fetch` is not available requests are sent through XHR, which supports standard `method`, `headers`, `credentials` and `body` settings; as seen in the code example above. 
+When requests are sent through `fetch` we can also set, in addition to the properties already mentioned, all the [options supported by fetch](https://developer.mozilla.org/en-US/docs/Web/API/fetch#options).
+
 ## Browser Support
 
 Tested in the latest Chrome, Firefox, Safari, Edge, and Internet Explorer 11. Requires a polyfill for TextEncoder/Decoder. Since only utf-8 encoding is required, it's recommended to use [text-encoding-utf-8](https://www.npmjs.com/package/text-encoding-utf-8) to minimize impact on bundle size.

src/fetch.js

@@ -1,11 +1,8 @@
 import { PatchResolver } from './PatchResolver';
 import { getBoundary } from './getBoundary';
 
-export function fetchImpl(
-    url,
-    { method, headers, credentials, body, onNext, onError, onComplete }
-) {
-    return fetch(url, { method, headers, body, credentials })
+export function fetchImpl(url, { onNext, onComplete, onError, ...fetchOptions }) {
+    return fetch(url, fetchOptions)
         .then((response) => {
             const contentType = (!!response.headers && response.headers.get('Content-Type')) || '';
             // @defer uses multipart responses to stream patches over HTTP