mod_proxy_http: fix load-balancer fallback for requests with a body.

mod_proxy_http: fix load-balancer fallback for requests with a body.

Since r1656259 (or r1656259 in 2.4.41) and the move of prefetch before connect,
the balancer fallback case where proxy_http_handler() is re-entered with the
next balancer member broke.

We need to save the body (partially) prefetched the first time and reuse it on
successive calls, otherwise we might forward partial or empty body.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1869216 13f79535-47bb-0310-9956-ffa450edef68