`cross-fetch` overrides `window.fetch` with non-standard behaviour

Bug description

When using solid-auth-fetcher in a modern browser, the used fetcher should be provided by the browser itself. However, it seems that the polyfill returned by cross-fetch does not default to window.fetch, and has non-standard behaviour.

What prompted reporting this bug specifically is the fact that the obtained Response doesn't have a .body method to access its content as a ReadableStream.

To Reproduce
Steps to reproduce the behavior:

1. If you don't have a React sandbox, create one (npx create-react-app my-app)
2. Install cross-fetch
3. Make sure useEffect is imported in App.js
4. import the cross-fetch fetcher: import {fetch as crossFetch} from "cross-fetch"
5. Add this code snippet to the App function:

useEffect(() => {
     const buildFetcher = async () => {
      let fetcher;
      if(typeof window !== undefined && typeof window.fetch !== undefined) {
        console.log("Using the window fetcher")
        fetcher = window.fetch;
      } else {
        console.log("Using cross-fetch")
        fetcher = crossFetch;
      }
      fetcher("https://ruben.verborgh.org/profile/")
      .then(response => response.body)
      .then(stream => stream.getReader())
      .then(reader => {
        return reader.read().then(function processText({ done, value }) {
          if (done) {
            console.log("Stream complete");
            return;
          }
          console.log(value);
          return reader.read().then(processText);
        });
      });
    }
    buildFetcher();
  });

5. Run the app: npm run start

Expected behavior

• The code prints "Using the window fetcher", and then the stream content (byte arrays). This is what happens with the provided code snippet.
• When replacing fetcher("https://ruben.verborgh.org/profile/") with crossFetch("https://ruben.verborgh.org/profile/"), to force using the polyfill, then an error occurs, "TypeError: stream is undefined", because the response returned by cross-fetch does not implement the body method.

Desktop (please complete the following information):

• OS: Ubuntu 18.04
• Browser: Firefox 77.0.1, Chrome 83

Additional context

LDflex expects the response to be readable as a stream. Also, this might have been the root cause of #88, and all the discussion around non-standard headers.

Start by reproducing the behavior in the App.js example with cross-fetch and the browser fetcher, then compare the returned Response.body and stream handling. Check the solid-auth-fetcher integration and the cross-fetch behavior described in the issue; done means the supported fetch path provides a readable response stream or the limitation is clearly documented and tested.