event_machine: avoid close on deferred response

event_machine: avoid close on deferred response

close_connection_after_writing only if not deferred, as in
cool.io

Deferred responses may buffer more data down the line, so
keep the connection alive if we have a deferred response
body.

[ew: clear @deferred when we really want to quit,
 updated commit message]

Acked-by: Eric Wong <normalperson@yhbt.net>