BUG/MINOR: tcpcheck: refresh the start-line after updating the authority

BUG/MINOR: tcpcheck: refresh the start-line after updating the authority

When an HTTP health-check request was built with an absolute URI, each
"host" header set by "http-check send" updated the authority of the URI with
http_update_authority(). This rewrote the start-line, which could move it
elsewhere in the HTX message. So, in tcpcheck_eval_send(), the start-line
pointer <sl> must be renewed in that case.

Let's retrieve the start-line again after each authority update. A reg-test
was added.

This must be backported to all supported versions.