Reduce code duplication between ssl_proxy_tunnel::handle_status_line()
and the method with the same name in asio_context itself by moving the
common handling of connections closed by server into a new function.

No real changes, this is a pure refactoring.

When an SSL connection times out due to being closed by server, a
different error code is returned, so we need to check for it too in
was_closed_by_server().

Without this, losing connection was not detected at all when using
HTTPS, resulting in "Failed to read HTTP status line" errors whenever
the same http_client was reused after more than the server keep alive
timeout of inactivity.

See #592.

Creating a new request when the existing connection being used was
closed by the server didn't work correctly because the associated input
stream was already exhausted, as its contents had been already "sent" to
the server using the now discarded connection, so starting a new request
using the same body stream never worked.

Fix this by explicitly rewinding the stream to the beginning before
using it again.

Note that even with this fix using a custom body stream which is not
positioned at the beginning initially (or which doesn't support
rewinding) still wouldn't work, but at least it fixes the most common
use case.

See #592.

…ad_status_line.

Avoid increasing public surface with rewind function.