mod_proxy_http: follow up to r1836588: fix drop of spurious 100 responses.

r1836588 broke t/security/CVE-2008-2364.t by forwarding more than one
"100 continue" response, fix it.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1836750 13f79535-47bb-0310-9956-ffa450edef68