correct behavior for https_proxy
This goes together with change to libwww-perl cb80c2ddb7
- new method _upgrade_sock in LWP::Protocol::https, but only if the
  default SSL library IO::Socket::SSL is used (e.g. can start_SSL).
  This enables LWP::Protocol::http to establish tunnel and upgrade an
  existing socket.
- extensive test t/https_proxy.t to make sure that LWP behaves
  correctly. test can be run with IO::Socket::SSL and Net::SSL as
  backend by forcing library with environment
  PERL_NET_HTTPS_SSL_SOCKET_CLASS