mod_proxy_http: follow up to r1836588/r1836648: handle unread 100-continue.

When the backend responds with a non-interim response to a 100-continue,
mod_proxy_http won't read the client's body, so make sure "Connection: close"
ends up being added to the response if nobody reads that body later.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1836716 13f79535-47bb-0310-9956-ffa450edef68