mod_proxy: Let fixup hooks know about the Host header (and eventually overwrite it).

mod_proxy: Let fixup hooks know about the Host header (and eventually overwrite it).

If proxy_run_fixups() sets a Host header there will be two ones sent to the
origin server.

Instead, let the hooks know about the Host by setting it in the r->headers_in
passed to proxy_run_fixups(), and use the actual value afterwards.
Note: if proxy_run_fixups() unsets the Host we'll keep ours.

Suggested by: rpluem

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1901485 13f79535-47bb-0310-9956-ffa450edef68