mod_proxy: Handle UDS URIs with empty hostname as if they had no hostname.

It was reported to me (privately) that r1893101 broke existing settings like:
  SetHandler "proxy:unix:///path/to/uds.sock|fcgi://localhost/"

RFC 3986 (section 3.2.2) says that:
   If the URI scheme defines a default for host, then that default
   applies when the host subcomponent is undefined or when the
   registered name is empty (zero length).  For example, the "file" URI
   scheme is defined so that no authority, an empty host, and
   "localhost" all mean the end-user's machine, whereas the "http"
   scheme considers a missing authority or empty host invalid.

Let's consider that the "unix" scheme is closer to the "file" scheme than
the "http" one, and accept "unix:///path/to/uds.sock" as a valid URI.



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1893519 13f79535-47bb-0310-9956-ffa450edef68
