BUG/MINOR: http: don't process abortonclose when request was sent

BUG/MINOR: http: don't process abortonclose when request was sent

option abortonclose may cause a valid connection to be aborted just
after the request has been sent. This is because we check for it
during the session establishment sequence before checking for write
activity. So if the abort and the connect complete at the same time,
the abort is still considered. Let's check for an explicity partial
write before aborting.

This fix should be backported to 1.4 too.