I am using libsoup 2.42.0, glib 2.36.0, and the new proxy-resolver feature to connect to tor/socks5 directly.  However, port 80 doesn't seem to work with xombrero/webkit.  I have written a small app to demonstrate.  

#include <libsoup/soup.h>

#include <stdio.h>
#include <stdlib.h>

int
main(void)
{
        GProxyResolver *r;
        SoupSession *s;
        SoupMessage *msg;
        guint status;
        int i;
        char *hosts[] = {
                "http://www.google.com",
                "https://www.google.com",
                "http://www.google.com:80",
                "http://labs.webmetrics.com:8080"
        };

        r = g_simple_proxy_resolver_new("socks5://127.0.0.1:9050", NULL);
        s = soup_session_new();
        g_object_set(s, "proxy-resolver", r, NULL);

        for (i = 0; i < sizeof(hosts) / sizeof(hosts[0]); i++) {
                msg = soup_message_new("GET", hosts[i]);
                status = soup_session_send_message(s, msg);
                printf("%s: soup:%s http:%d\n", hosts[i],
                        soup_status_get_phrase(status), msg->status_code);
        }
        return (0);
}

http://www.google.com: soup:Cannot connect to destination http:4
https://www.google.com: soup:OK http:200
http://www.google.com:80: soup:Cannot connect to destination http:4
http://labs.webmetrics.com:8080: soup:OK http:200

On the first and third test (port 80 tests), Tor says:
 Tor[17353]: Application asked to connect to port 0. Refusing.

Seems like libsoup or gio is ignoring setting the port number if it is supposed to be 80?