fix: handle Immutable Response objects

[runspired]

Response headers returned by fetch are both readonly and Immutable:

• listed as readonly https://developer.mozilla.org/en-US/docs/Web/API/Response/headers
• spec lists it as immutable https://fetch.spec.whatwg.org/#concept-headers-guard

This clones the response, allowing us to adjust the response headers, but does so in a way that passes the stream through.