HTTP socket engine: support newer HTTP proxies

After sending authentication, we need to revert all states to be able
to read the HTTP header again. Before, we would not try to read an
HTTP header after sending authentication.

Change-Id: Id4b95eda9881a37bcfbae0570756bb3e4918a568
Reviewed-by: Richard J. Moore <rich@kde.org>
Reviewed-by: Shane Kearns <shane.kearns.qt@gmail.com>