When an HTTPS response carries a Strict-Transport-Security header, the
received policy is now respected. Subsequent HTTP requests to a known
HSTS host are upgraded to HTTPS before the fetch algorithm makes
further decisions such as CORS and mixed content.
Fixes tpexpress.co.uk, where an XHR redirects HTTPS -> HTTP -> HTTPS,
relying on a HSTS policy received on the document response to avoid the
CORS failure.