mod_proxy: follow up to r1895921: Don't prevent forwarding URIs w/ no hostname.

mod_proxy: follow up to r1895921: Don't prevent forwarding URIs w/ no hostname.

r1895921 changed proxy_detect() to disable forward proxying for URIs with no
hostname which is wrong, there might exist a third-party proxy module handling
the "urn:" scheme for instance (thanks Roy for the catch!).

For this to work, we also need to leave the forward proxied URI alone in
ap_proxy_pre_request() with no UDS special case or alike, a proxy module can
then catch (or not) the original URI as expected.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1895981 13f79535-47bb-0310-9956-ffa450edef68