Handle redirects from responses

[RangerMauve]

We should support fetch handlers returning 301 and 302 redirects and support the redirect parameter in the request.

The default redirect parameter should be follow, but the redirect following should be opted out by redirect: "manual".

This makes some logic like navigating to a proper directory URL ending in / more straightforward to implement. Similarly, being able to opt-out is useful for the Browser protocol handler use case where the user agent follows redirects for you.