Proxy: fixed incorrect URI change due to if (ticket #86).

Proxy: fixed incorrect URI change due to if (ticket #86).

In the following configuration request was sent to a backend without
URI changed to '/' due to if:

    location /proxy-pass-uri {
        proxy_pass http://127.0.0.1:8080/;

        set $true 1;

        if ($true) {
            # nothing
        }
    }

Fix is to inherit conf->location from the location where proxy_pass was
configured, much like it's done with conf->vars.