For each call to fetch, set the User-Agent header

[shawwn]

Closes #1230

For each call to fetch in src/DOM.js, this PR sets the User-Agent header to window.navigator.userAgent, where window is defined as this.ownerDocument.defaultView.

This seems to be the correct way to get the window associated with the dom element in a multi-window environment, and window.navigator.userAgent seems to be the correct value to pass.

[avaer]

Per avaer/window-fetch#2, I don't think adding manual options everywhere is the right resolution.

[shawwn]

Suppose the intercept feature is done.

At that point, you'd still need to decide what User-Agent value to pass to fetch. And that decision would presumably be made somewhere other than inside the DOM element, just before the call to fetch.

In a multi-window multi-worker environment, it's not possible to know what the value of User-Agent should be for any arbitrary DOM element. The correct User-Agent value is window.navigator.userAgent. However, given DOM element X and DOM element Y, then X and Y might have completely different values for this.ownerDocument (and hence, different values for their corresponding window.navigator.userAgent).

Suppose User-Agent is a static unchanging value, knowable at startup time. Then in that case, simply setting something like

const _fetch = fetch;
fetch = (url, opts, ...args) => {
  let h = (opts && opts.headers) ? opts.headers : new Headers();
  if (!(h instanceof Headers)) { h = new Headers(h); }
  if (!h.has('User-Agent')) {
   h.set('User-Agent', `User-Agent: ${ExokitUserAgent}`);
  }
  return _fetch(url, {... (opts || {}), headers: h}, ...args);
}

at the top of src/DOM.js would give you the equivalent functionality as the intercept API. But it's ugly compared to just passing in a value.

[avaer]

The purpose of interception is to generalize the above case into a central control point.

That includes:

• Different fetch callpoints
• XMLHttpRequest
• User fetch
• Various Workers

[RangerMauve]

Yeah, I agree with @modulesio. This is a really great idea and we should be setting the header, but it'd be better to do it inside the fetch implementation rather than in the code invoking fetch.

[shawwn]

Closing in favor of #1237