- Refactors some h2mux specific logic from connection/header.go to connection/h2mux_header.go
- Do the same for the unit tests
- Add a non-h2mux "is control response header" function (we don't need one for the request flow)
- In that new function, do not consider "content-length" as a control header
- Use that function in the non-h2mux flow for response (and it will be used also in origintunneld)