[fetch] referrer same-origin constraint is a footgun for people trying to "copy" a Request (#245)

Recently I saw a website in the wild attempting to do this:

```
var request(newURL, {
  referrer: oldRequest.referrer,
  // copy other attributes as well
});
```

This will work just fine during development on localhost, because .referrer will most likely always be same-origin.  When the site is posted on twitter, for example, it will be visited through a t.co redirector.  This results in a t.co referrer which is cross-origin.

So the site that worked fine in local development will blow up when its published to twitter.  This seems like a bit of a footgun.

We could make new Request() silently ignore the value if its invalid instead of throwing.  This is somewhat similar to using bad header values.  They just get ignored.

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Reply to this email directly or view it on GitHub:
https://github.com/whatwg/fetch/issues/245