PR53420: Proxy responses with error status and
"ProxyErrorOverride On" hang until proxy timeout.

Regression from 2.2. It was introduced by r912063
in order to fix PR41646.


Switch preference for headers, Transfer-Encoding
first, Content-Length second.

Addition to r1615289.

Submitted by: rjung
Reviewed/backported by: jim


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1621601 13f79535-47bb-0310-9956-ffa450edef68