mod_proxy_http: don't pretend we are sending a body before ProxyErrorOverride

takes place, the response it to be handled by ErrorDocument is this case.
Also it useless to take care of pre-read data since the body is discarded in
this case.



git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1754974 13f79535-47bb-0310-9956-ffa450edef68