If Fetch API is specified as a polyfill in settings, then Headers, Request, and Response should also assume to be polyfilled

Headers, Request, and Response are part of the Fetch API spec, therefore if `fetch` is specified as a polyfill, then we should assume that these other three are polyfilled as well (similar to specifying `Promise` and `Promise.resolve` and `Promise.reject` are assumed to be polyfilled, as is working in the `@next` release currently).

Ref:
* https://developer.mozilla.org/en-US/docs/Web/API/Fetch_API
* https://developer.mozilla.org/en-US/docs/Web/API/Headers
* https://developer.mozilla.org/en-US/docs/Web/API/Request
* https://developer.mozilla.org/en-US/docs/Web/API/Response