Support streaming requests

[Arman Bilge]

https://developer.chrome.com/articles/fetch-streaming-requests/

This will require some sort of feature test:

 const supportsRequestStreams = (() => {
    let duplexAccessed = false;
    
    const hasContentType = new Request('', {
      body: new ReadableStream(),
      method: 'POST',
      get duplex() {
        duplexAccessed = true;
        return 'half';
      },
    }).headers.has('Content-Type');
    
    console.log({ duplexAccessed, hasContentType });
    
    return duplexAccessed && !hasContentType;
  })();

[Arman Bilge]

Crap, that feature test might only work for Chrome.

• whatwg/fetch#1470