Make 'immutable' cache-control value HTTP/3 QPACK compatible

At the moment passing an `immutable: true` option adds an immutable value to the `Cache-Control` header. Ideally the header would be put as a separate `Cache-Control` header entry making such request QPACK-compatible.

> Note: When evaluating the use of immutable and QPACK: If you're concerned that immutable changes the predefined value provided by QPACK, consider that in this case, the immutable part can be encoded separately by splitting the Cache-Control value into two lines — though this is dependent on the encoding algorithm a particular QPACK implementation uses.
([MDN reference](https://developer.mozilla.org/en-US/docs/Web/HTTP/Caching#validation_2))