MINOR: http-ana: Match on the path if the monitor-uri starts by a /

MINOR: http-ana: Match on the path if the monitor-uri starts by a /

if the monitor-uri starts by a slash ('/'), the matching is performed against
the request's path instead of the request's uri. It is a workaround to let the
HTTP/2 requests match the monitor-uri. Indeed, in HTTP/2, clients are encouraged
to send absolute URIs only.

This patch is not tagged as a bug, because the previous behavior matched exactly
what the doc describes. But it may surprise that HTTP/2 requests don't match the
monitor-uri.

This patch may be backported to 2.1 because URIs of HTTP/2 are stored using the
absolute-form starting this version. For previous versions, this patch will only
helps explicitely absolute HTTP/1 requests (and only the HTX part because on the
legacy HTTP, all the URI is matched).

It should fix the issue #509.

(cherry picked from commit 6072beb214d04ee2344872e100f4fdc480dd2496)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>