Disable HTTP 1.0

You should probably get rid of all the HTTP::header stuff, but yes that should work.

when HTTP_REQUEST {
    if { not ( [HTTP::header exists Host] ) } {
        HTTP::header insert Host [IP::client_addr]
    }
    HTTP::version "1.1"
}
when HTTP_RESPONSE {
    HTTP::version "1.1"
}

This will probably require some tweaking in your environment. Also note that this will change the protocol version as it passes through the proxy. The requests and responses will arrive at the client with 1.1, but there's nothing you can do to prevent either party from continuing to use 1.0.