SimpleHttp IPv6 support

[tthebst]

https://github.com/apoelstra/rust-jsonrpc/blob/master/src/simple_http.rs#L302

Url parsing currently fails for IPv6 addresses.

My suggestion would be to sth simpler like this. Instead of trying to split the string.

// Needs to be of format <host_name>:<port>
let mut addr = match after_auth.to_socket_addrs() {
    Ok(addr) => addr,
    Err(_) => {
        // Try to add fallback port
        format!("{}:{}", after_auth, fallback_port).to_socket_addrs()?
    }
};

[apoelstra]

This seems like it'd be worth a shot. I seem to remember trying this a long time ago and finding that passing strings directly to to_socket_addrs didn't work for some reason.