BUG/MEDIUM: http/htx: unbreak option http_proxy

BUG/MEDIUM: http/htx: unbreak option http_proxy

The temporary connection used to hold the target connection's address
was missing a valid target, resulting in a 500 server error being
reported when trying to connect to a remote host. Strangely this
issue was introduced as a side effect of commit 2c52a2b9e ("MEDIUM:
connection: make mux->detach() release the connection") which at
first glance looks unrelated but solidly stops the bisection (note
that by default this part even crashes). It's suspected that the
error only happens when closing and destroys pending data in fact.

Given that this feature was broken very early during 1.8-rc1 development
it doesn't seem to be used often. This must be backported as far as 1.8.