MINOR: checks: Keep the Host header and the request uri synchronized

MINOR: checks: Keep the Host header and the request uri synchronized

Because in HTTP, the host header and the request authority, if any, must be
identical, we keep both synchornized. It means the right flags are set on the
HTX statrt-line calling http_update_host(). There is no header when it happens,
but it is not an issue. Then, if a Host header is inserted,
http_update_authority() is called.

Note that for now, the host header is not automatically added when required.