Forward-port the comments from commit 01e2920 (v0.10) to the master
branch. Everything else from that patch already exists in master.
It didn't merge cleanly because lib/http.js has been split up in
several files.
Fixes #3740
In the case of pipelined requests, you can have a situation where
the socket gets destroyed via one req/res object, but then trying
to destroy *another* req/res on the same socket will cause it to
call undefined.destroy(), since it was already removed from that
message.
Add a guard to OutgoingMessage.destroy and IncomingMessage.destroy
to prevent this error.
