http: don't requeue if using write buffer

http: don't requeue if using write buffer

The write buffering will already be processed inside
->event_step, so requeue will cause a needless read(2) outside
of epoll_wait/kevent(2) readiness notifications.

This ought to avoid problems in case of pipelined connections,
but those aren't possible behind a reverse proxy and AFAIK most
HTTP clients don't do pipelining.  This bug was only noticed via
strace while searching for extra syscalls, and not from
real-world use.