cypress: Bug: experimentalFetchPolyfill doesn't handle new Request API

Current behavior:

When constructing a request using fetch’s new experimental polyfill and the new Request API, we send [object Request] in the URL’s path. When the experimental polyfill is off, we correctly stringify the URL’s path.

Desired behavior:

Should behave like XHR would when dealing with the new Request API

Test code to reproduce

In https://github.com/cypress-io/cypress-test-tiny/tree/query-string if you change the ajax.html fixture to use an example from MDN’s site then you’ll see the above screenshot.

const myHeaders = new Headers();

const myRequest = new Request('flowers.jpg?size=300', {
  method: 'GET',
  headers: myHeaders,
  mode: 'cors',
  cache: 'default',
});

fetch(myRequest)
  .then(()) => console.log(...arguments))

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