Referrer-Policy header should support multiple values #882
Description
In order to support fallback policies when a subset of the full policies is supported (such as when a new policy is introduced in the specification), hyper's Referrer-Policy header should store the most recent value that is successfully parsed from the list of potential multiple values, per https://w3c.github.io/webappsec-referrer-policy/#parse-referrer-policy-from-header .