MEDIUM: http-htx: Keep the Host header and the request start-line synchronized

MEDIUM: http-htx: Keep the Host header and the request start-line synchronized

In HTTP, the request authority, if any, and the Host header must be identical
(excluding any userinfo subcomponent and its "@" delimiter). So now, during the
request analysis, when the Host header is updated, the start-line is also
updated. The authority of an absolute URI is changed accordingly. Symmetrically,
if the URI is changed, if it contains an authority, then then Host header is
also changed. In this latter case, the flags of the start-line are also updated
to reflect the changes on the URI.