BUG/MINOR: http_fetch: Fix http_auth/http_auth_group when called from TCP rules

BUG/MINOR: http_fetch: Fix http_auth/http_auth_group when called from TCP rules

These sample fetches rely on the static fnuction get_http_auth(). For HTX
streams and TCP proxies, this last one gets its HTX message from the request's
channel. When called from an HTTP rule, There is no problem. Bu when called from
TCP rules for a TCP proxy, this buffer is a raw buffer not an HTX message. For
instance, using the following TCP rule leads to a crash :

  tcp-request content accept if { http_auth(Users) }

To fix the bug, we must rely on the HTX message returned by the function
smp_prefetch_htx(). So now, the HTX message is passed as argument to the
function get_http_auth().

This patch must be backported to 2.0 and 1.9.