Merge r1507320 from trunk:

1-arg SetEnv is valid

http://httpd.apache.org/docs/2.2/mod/mod_proxy_http.html#comment_1528




git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1507321 13f79535-47bb-0310-9956-ffa450edef68