When `free`ing the socket to be reused in keep-alive Agent wait for
both `prefinish` and `end` events. Otherwise the next request may be
written before the previous one has finished sending the body, leading
to a parser errors.
PR-URL: https://github.com/nodejs/node/pull/7149
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>