Call soup_message_clean_response() when restarting a message

Call soup_message_clean_response() when restarting a message

When a message got restarted, we were leaving the previous response
state in the message, which is bad for various reasons.

In particular, this caused a problem with non-keepalive redirections
of https URLs through proxies (!), because after the second CONNECT
succeeded, it would see that the message already had a status_code
set, and so it thought the message had been cancelled or something
while it was processing the CONNECT. proxy-test now has a regression
test for this case.

Based on patches and analysis from DongJae Kim and Thierry Reding.

https://bugzilla.gnome.org/show_bug.cgi?id=631368