[MEDIUM] http: forward client's close when abortonclose is set

[MEDIUM] http: forward client's close when abortonclose is set

While it's usually desired to wait for a server response even
when the client closes its request channel, it can be problematic
with long polling requests. In order to let the server decide what
to do in such a case, if option abortonclose is set, we simply
forward the shutdown to the server. That way, it can decide to
take the appropriate action. Most servers will still process the
request, while some will probably want to abort.

Obviously, this only works as long as the client has not sent
another pipelined request over the same connection.

(was commit 0e25d86da49827ff6aa3c94132c01292b5ba4854 in 1.4)