Strips the Referer header down to an origin when it's size exceeds 4k.
As noted in https://github.com/xsleaks/xsleaks/wiki/Browser-Side-Channels#cache-and-error-events, servers will often behave in unexpected ways when presented with an overly-long `Referer` header. This is unfortunate, as `Referer` is one header whose length attackers generally retain control over when generating `no-cors` requests.
Docs: https://github.com/whatwg/fetch/issues/903
Explainers: https://github.com/whatwg/fetch/issues/903