check if the upstream is a placeholder

[jasonmccallister]

This removes the hard coded port 80 when the reverse_proxy upstream is using a placeholder.

Here is an example Caddyfile to reproduces the issue:

my.domain.com {
    encode zstd gzip

    some_module_that_sets_the_http_var

    respond /test 200 { # works as expected
    	body {http.vars.some_custom_var}
    	close
    }

    tls internal

    reverse_proxy / {
        to {http.vars.some_custom_var}
    }

    file_server
}

More detailed explanations are documented in this conversation: https://caddy.community/t/dynamically-set-reverse-proxy-upstreams-from-custom-module/10142/15

[CLAassistant]

All committers have signed the CLA.

[mholt]

I actually couldn't think of a better way to do this... but I just want a comment here to explain why this condition is what it is.

Thanks!

[mholt]

Anyway, thank you for the fix! I will merge this in a moment.