Honor on_unsupported_protocol for intercepted https_port (#689)

Honor on_unsupported_protocol for intercepted https_port (#689)

... when Squid discovers a non-TLS client while parsing its handshake.

For https_port traffic, ConnStateData::switchToHttps() relies on start()
to set preservingClientData_ correctly, but shouldPreserveClientData(),
called by start() to set preservingClientData_, was not preserving TLS
bytes in the https_port start() context. Typical debug messages:

    parseTlsHandshake: Got something other than TLS ... Cannot SslBump
    tunnelOnError: may have forgotten client data; send error: 40