Propagate Proxy-Authorization header correctly

PR 25947
RFC2616 tells us:
  (1) If we haven't authenticated, we must pass the header on.
  (2) If we have authenticated, we MAY pass it on.
I've made the latter case configurable by ENV(Proxy-Chain-Auth).

Also, Proxy-Authenticate is a response header, and doesn't belong
in a check of request headers.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@574021 13f79535-47bb-0310-9956-ffa450edef68