BUG/MINOR: http-htx: Do case-insensive comparisons on Host header name

BUG/MINOR: http-htx: Do case-insensive comparisons on Host header name

When a header is added or modified, in http_add_header() or
http_replace_header(), a comparison is performed on its name to know if it is
the Host header and if the authority part of the uri must be updated or
not. This comparision must be case-insensive.

This patch should fix the issue #522. It must be backported to 2.1.

(cherry picked from commit 3e1f7f4a39456051873aec75eb7d9e8f4ace6813)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>