BUG/MINOR: httpclient: allow to replace the host header

BUG/MINOR: httpclient: allow to replace the host header

This patch allows to replace the host header generated by the
httpclient instead of adding a new one, resulting in the server replying
an error 400.

The host header is now generated from the uri only if it wasn't found in
the list of headers.

Also add a new request in the VTC file to test this.

This patch must be backported in 2.5.

(cherry picked from commit f03b53c81d52344c29d7a5434ac32f3127cebf75)
Signed-off-by: William Lallemand <wlallemand@haproxy.org>