Merge r527969 from trunk:

Merge r527969 from trunk:

HTTP proxy ProxyErrorOverride: Leave 1xx and 3xx responses alone.  Only
processing of error responses (4xx, 5xx) will be altered.

PR: 39245

This is based on a patch submitted by Bart van der Schans <schans hippo.nl>
and tweaked slightly by me based on discussions on dev@ since April 2006.
I think rpleum was the first to mention the 1xx issue.

Submitted by: trawick
Reviewed by: jim

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.2.x@534068 13f79535-47bb-0310-9956-ffa450edef68